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 cross sections 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.
Highlights
Rivers and submarine channels carry and distribute sediment, organic matter, pollutants around the world (Hodgson et al, 2018; Kane and Clare, 2019), and it is of paramount importance to understand the dynamics controlling sediment distribution in these channelized depositional systems
While Dorrell et al (2015) demonstrate that aggrading submarine channels eventually become unstable and avulse due to geometric constraints, we suggest that submarine channel-floodplain coupling is stronger than that in rivers, and as a result submarine channels are more stable than rivers under aggradational conditions
This vertical stratification limits the potential for avulsion because: (1) the dense, energetic, coarse-grained portion of the flow remains near the channel thalweg rather than near the levee crest (Hansen et al, 2015; Jobe et al, 2017), and (2) the top of the flow has very low sediment concentration and density contrast with the ambient fluid (Imran et al, 1998) and little to no driving force for erosion as compared to rivers (Konsoer et al, 2013), suggesting there is diminished ability for turbidity currents to erode the external levee and trigger an avulsion
Summary
Rivers and submarine channels carry and distribute sediment, organic matter, pollutants around the world (Hodgson et al, 2018; Kane and Clare, 2019), and it is of paramount importance to understand the dynamics controlling sediment distribution in these channelized depositional systems. The channel belt encompasses all channelrelated sediment in one avulsion cycle (thalweg, bar, splay, internal and external levee, and floodplain deposits; Figures 1, 2). Aggradation of the channel above the floodplain results in storage of potential energy in the flow to cause an avulsion (Imran et al, 1998; Mohrig et al, 2000). In submarine channels sculpted by turbidity currents, the potential energy is much lower due to the relatively small excess density between a turbidity current and the ambient seawater, leading to taller external levees and less frequent avulsion (Imran et al, 1998). The amount of pre-avulsion aggradation possible in submarine channel-belts has never been quantified
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