Basin water depth (h) governs the long‐term morphodynamics of river deltas, which are embodied in the grade index (Gindex). The Gindex, a volume‐in‐unit‐time ratio of subaerial sediment allocation to the entire supplied sediment, can be given as a function of the dimensionless basin water depth (h*). Tank experiments reported herein reveal that delta progradation and deltaplain aggradation are suppressed and distributary channel migration and avulsion take place less frequently when the Gindex value is lower (i.e., when the basin water is deeper; h* ≫ 1). If the Gindex~0 (i.e., extremely deep basin water; h*~+∞), the delta can neither prograde nor aggrade, and the distributary channels tend to stabilize. The grade index model helps explain the contrasting morphodynamics of the Liwu Delta (east Taiwan) and Yellow River Delta, as natural examples of deepwater and shallow‐water deltas, respectively.

Grade index (Gindex) is a dimensionless number given as the volume‐in‐unit‐time ratio of subaerial allocation to both subaerial and subaqueous allocations of sediment supplied to a delta from upstream. It was originally proposed for understanding the effect of basin water depth on the morphodynamics of delta distributary channels under stationary relative sea level. We here examine how rising relative sea level modulates the Gindex, using geometrical reasoning and numerical simulations. We find that the grade index model can account for autoretreat of the deltaic shoreline, autodrowning of the whole system, and autobreak of the deltaic sedimentation, all of which are the consequences of autogenic nonequilibrium responses to steadily rising relative sea level. The regressive‐to‐transgressive threshold (i.e. the onset of autoretreat) is crossed when the delta plain's dimensionless basal area (At*) encounters a critical value that is expressed in terms of Gindex: regression and transgression are sustained when At* is below and above the threshold, respectively. The mode of transgression depends on the slope conditions. If the hinterland slope (γ) is steeper than the foreset slope (β), both At* and Gindex decrease as the relative sea‐level rises. Eventually, the depositional system experiences autodrowning when At* = Gindex = 0. If γ < β; on the other hand, both At* and Gindex increase. This latter slope condition eventually causes autobreak of the deltaic sedimentation, afterward of which At* = Gindex = 1. The grade index model is useful for interpreting and predicting the stratigraphic responses of natural deltaic clinoforms in conditions of rising relative sea level.

Submarine channels act as the main conduits for the transport of sediment to deep-water basins by sediment gravity flows. The interplay between fault-related deformation and the initiation and development of the channels is poorly known.Here, we present the identification, formation and evolution of the Miocene slopeparallel channel by employing 3D seismic reflection, wireline-log and core data in the eastern slope of Yinggehai Basin, South China Sea. Based on the lengths and plan-view shapes, a total of three different types of fault-associated slope-parallel depressions have been identified. The depressions were formed in the fault zone and controlled by the reactivation of the underlying older faults. Among them, Type-1 depressions are short (<20 km) oval or circle shaped possessing only one depocenter. Type-2 depressions are elongated (25-70 km), and usually have multiple depocenters. Type-3 depressions, which are usually connected by slopeperpendicular channels in the head and middle, are longer (more than 190 km) and connect shallow and deep-water basins. The analysis of morphology, erosivity and material transport shows that Type-3 depressions are fully fledged channels.Type-1 and Type-2 depressions are channel precursors representing the initial stage of channel evolution. With this motive, a model for the initiation and evolution of slope-parallel submarine channels controlled by strike-slip-extensional faults is presented. Unlike the previous investigations which suggest that erosion