Varying discharge controls on timescales of autogenic storage and release processes in fluvio‐deltaic environments: Tank experiments

Abstract

Changes in external forcing have traditionally been the main areas of interest in understanding sedimentary records, while in most stratigraphic interpretation, autogenic behavior has been thought of as a “noise” generator. This study aims to investigate autogenic processes in a fluvio‐deltaic system under a range of discharge conditions and to show that autogenic processes generate distinct signatures rather than random noise. A matrix of nine different experiments is presented here to systematically evaluate the effects of sediment and water discharge variations on the timescale of fluvial autogenic processes. Temporary sediment storage regularly occurs by backfilling of sediment in the fluvio‐deltaic channels, followed by a period of strong channelization that releases the stored sediment. These storage and release processes cycle along with changes in the fluvial slope and planform pattern of the flow. Here we propose that the autogenic behavior of deltaic progradation has a distinct timescale that is controlled by sediment and water discharges. An increase in sediment discharge primarily reduces the autogenic timescale as higher sediment supply fills the channels faster. In contrast, the high sediment discharge causes a morphologic feedback by increasing the magnitude of fluvial slope change between the storage and release events and increasing the size of the temporary sediment storage (termed “the fluvial envelope”). This works against the sediment discharge control as the autogenic timescale is increased. Increasing the water discharge increases the autogenic timescale by improving the fluvial organization toward a strongly channelized system. Changes in autogenic timescale due to variations in the sediment and water discharges are nonlinear for different sediment to water discharge ratios. As the ratio decreases, the fluvial system is better organized and the timescale is more linearly related to the change in sediment discharge. As the ratio increases, deltas develop poorly organized fluvial systems and the associated timescales converge even with different sediment discharges. The results presented here provide enhanced interpretation of sedimentary records by better decoupling of autogenic signatures from allogenic products developed across a wide range of discharge conditions.