Search for: All records

ABSTRACT Buried channel sand bodies are important reservoirs of subsurface water and energy resources, but their arrangement and interconnectedness are difficult to predict. The dominant process that distributes channels and their sediments in alluvial basins is river avulsion, which occurs when a channel seeks a new location on the adjacent floodplain. Floodplain sedimentation, incision, and channel levee growth influence channel pathfinding during avulsion, and should control key aspects of the stratigraphic arrangement of channel bodies, including compensational (spatially and temporally even) deposition, stratigraphic completeness, and facies distributions; however, this impact has been difficult to isolate in natural and experimental basin fills. To test how different avulsion pathfinding parameters influence stratigraphic architecture, we use a numerical model of a fluvial fan to produce synthetic fluvial stratigraphy under seven different runs with progressively more complex channel pathfinding rules. In the simplest models where pathfinding is set by a random walk, the channel rapidly changes position and avulsions spread across the fan surface. The corresponding deposit is dominated by channel facies, is relatively incomplete, and the compensation timescale is short. As rules for pathfinding become more complex and channels can be attracted or repulsed by pre-existing channels, lobe switching emerges. Deposits become more diverse with a mix of channel and floodplain facies, stratigraphic completeness increases, and the compensation timescale lengthens. Previous work suggests that the compensation timescale is related to the burial timescale and relief across the depositional surface, yet we find that compensation approaches the burial timescale only for model runs with high morphodynamic complexity and relatively long topographic memory. Our results imply that in simple systems with limited degrees of freedom, the compensation timescale may become detached from the burial timescale, with uniform sedimentation occurring quickly relative to long burial timescales. 

Abstract. We investigate the interaction of fluvial and non-fluvial sedimentation on the channel morphology and kinematics of an experimental river delta. We compare two deltas: one that evolved with a proxy for non-fluvial (“marsh”) sedimentation (treatment experiment) and one that evolved without the proxy (control). We show that the addition of the non-fluvial sediment proxy alters the delta's channel morphology and kinematics. Notably, the flow outside the channels is significantly reduced in the treatment experiment, and the channels are deeper (as a function of radial distance from the source) and longer. We also find that both the control and treatment channels narrow as they approach the shoreline, though the narrowing is more pronounced in the control compared to the treatment. Interestingly, the channel beds in the treatment experiment often exist below sea level in the terrestrial portion of the delta top, creating a ∼ 0.7 m reach of steady, non-uniform backwater flow. However, in the control experiment, the channel beds generally exist at or above relative sea level, creating channel movement resembling morphodynamic backwater kinematics and topographic flow expansions. Differences between channel and far-field aggradation produce a longer channel in-filling timescale for the treatment compared to the control, suggesting that the channel avulsions triggered by a peak in channel sedimentation occur less frequently in the treatment experiment. Despite this difference, the basin-wide timescale of lateral channel mobility remains similar. Ultimately, non-fluvial sedimentation on the delta top plays a key role in the channel morphology and kinematics of an experimental river delta, producing channels which are more analogous to channels in global river deltas and which cannot be produced solely by increasing cohesion in an experimental river delta. 

Abstract We present the first investigation of subsidence due to sediment compaction and consolidation in two laboratory‐scale river delta experiments. Spatial and temporal trends in subsidence rates in the experimental setting may elucidate behavior which cannot be directly observed at sufficiently long timescales, except for in reduced scale models such as the ones studied. We compare subsidence between a control experiment using steady boundary conditions, and an otherwise identical experiment which has been treated with a proxy for highly compressible marsh deposits. Both experiments have non‐negligible compactional subsidence rates across the delta‐top, comparable in magnitude to our boundary condition relative sea level rise rate of 250 μm/hr. Subsidence in the control experiment (on average 54 μm/hr) is concentrated in the lowest elevation (<10 mm above sea level) areas near the coast and is likely related to creep induced by a rising water table near the shoreface. The treatment experiment exhibits larger (on average 126 μm/hr) and more spatially variable subsidence rates controlled mostly by compaction of recent marsh deposits within one channel depth (∼10 mm) of the sediment surface. These rates compare favorably with field and modeling based subsidence measurements both in relative magnitude and location. We find that subsidence “hot spots” may be relatively ephemeral on longer timescales, but average subsidence across the entire delta can be variable even at our shortest measurement window. This suggests that subsidence rates over a short time frame may exceed thresholds for marsh platform drowning, even if the long term trend does not. 

Abstract The low temporal completeness of fluvial strata could indicate that recorded events represent unusual and extreme conditions. However, field observations suggest that preserved strata predominantly record relatively common transport conditions—a paradox termed thestrange ordinarinessof fluvial strata. We theorize that the self‐organization of fluvial systems into a morphodynamic hierarchy that spans bed to basin scales facilitates the preservation of ordinary events in fluvial strata. Using a new probabilistic model and existing field and experimental data sets across these scales, we show that fluvial morphodynamic hierarchy enhances the stratigraphic preservation of medial topography—ordinary events. We show that lower‐order landforms have a higher likelihood of complete preservation when the kinematic rates of evolution of successive levels in the morphodynamic hierarchy are comparable. We highlight how relative changes in kinematic rates of evolution of successive levels in the morphodynamic hierarchy can manifest as major shifts in stratigraphic architecture through Earth history.