Comparing Aggradation, Superelevation, and Avulsion Frequency of Submarine and Fluvial Channels

Abstract

Constraining the avulsion dynamics of rivers and submarine channels is essential for predicting the distribution of sediment, organic matter, and pollutants in alluvial, deltaic, and submarine settings. We create a geometric channel-belt framework relating channel, levee, and floodplain stratigraphy that allows comparative analysis of avulsion dynamics for rivers and submarine channels. We utilize 52 channel-overbank crosssections within this framework to provide avulsion criteria for submarine channels, and how they differ from rivers. Superelevation and a new channel-floodplain coupling metric are two key parameters that control channel-belt thickness in both rivers and submarine channels. While rivers only superelevate 1 channel depth above the floodplain prior to avulsion, submarine channels are more stable during aggradation, with superelevation values commonly > 3 channel depths. Additionally, channel-floodplain coupling in rivers is often weak, with floodplain aggradation negligible compared to channel aggradation, making rivers avulsion-prone. However, floodplain aggradation is more significant for submarine channels, resulting in stronger channel-floodplain coupling and thus a decreased potential for avulsion. The combination of enhanced superelevation and strong channel-floodplain coupling results in submarine channel-belts that can be as thick as ∼10 channel depths, while fluvial channel belts are limited to 2 channel depths. Submarine channels are more stable because turbidity currents have ∼50x lower density contrast between flow and ambient fluid as compared to rivers. This density contrast creates far less potential energy for avulsion, despite the much greater relief of submarine levees compared to fluvial levees. The modern Amazon submarine channel showcases this stability, with a channel belt that is ∼5 channel-depths thick for more than 400 streamwise km, which is more than twice the superelevation that a river is capable of. We interpret that enhanced floodplain aggradation and levee aggradation (and thus superelevation) in submarine channel belts are promoted by unique submarine flow characteristics, including turbidity current overspill, flow-stripping, and hemipelagic processes. We emphasize that rivers and submarine channels display very different avulsion dynamics and frequencies, profoundly affecting the stratigraphic architecture of channel-belt and downstream distributary deposits.

Morphodynamic Hierarchy and the Fabric of the Sedimentary Record

“…For example, deepwater systems, crafted by turbidity currents, exhibit hierarchical organization into dunes, bars, channels, and channel belts (Wynn et al, 2007). While important differences exist between channelized systems in deepwater and terrestrial environments (Jobe et al, 2016(Jobe et al, , 2020Wynn et al, 2007), our results suggest that stratigraphic preservation in deepwater environments is also likely controlled by the relative scales and rates of evolution of successive levels in the morphodynamic hierarchy.…”

Times Associated With Source-to-Sink Propagation of Environmental Signals During Landscape Transience

“…In the distal part of a marine SRSs, sediment can be buried in submarine channels by processes similar to terrestrial channels (Figure 5). Although the dynamics and frequencies of lateral (avulsion and lateral migration) and vertical (incision vs. aggradation) submarine channel movements differ from terrestrial rivers (e.g., Jobe et al, 2020), the general effect of lateral and vertical submarine channel movement on sediment transport times should be analogous to terrestrial river dynamics explained above. Different to terrestrial rivers are sediment deposition processes related to turbidity currents within submarine channels.…”

Intra-oceanic submarine arc evolution recorded in an ~1-km-thick rear-arc succession of distal volcaniclastic lobe deposits

“…Autocyclic switching would result in alternating lobe-axis (coarser) and lobe-margin (finer) sedimentation. In the case of vertical aggradation of material, the coarse-fine alternation in the unit III stratigraphic column would be a function of allocyclic changes (pulses) in supply of coarse sediment not associated with channel avulsion (e.g., Jobe et al, 2020). The allocyclic pulses might result from the tempo of magmatic processes in the case of this magmatic arc example.…”