Initiation and evolution of knickpoints and their role in cut-and-fill processes in active submarine channels

Abstract

Submarine channels are the main conduits and intermediate stores for sediment transport into the deep sea, including organics, pollutants, and microplastics. Key drivers of morphological change in channels are upstream-migrating knickpoints whose initiation has typically been linked to episodic processes such as avulsion, bend cutoff, and tectonics. The initiation of knickpoints in submarine channels has never been described, and questions remain about their evolution. Sedimentary and flow processes enabling the maintenance of such features in non-lithified substrates are also poorly documented. Repeated high-resolution multibeam bathymetry between 2012 and 2018 in the Capbreton submarine canyon (southeastern Bay of Biscay, offshore France) demonstrates that knickpoints can initiate autogenically at meander bends over annual to multi-annual time scales. Partial channel clogging at tight bends is shown to predate the development of new knickpoints. We describe this initiation process and show a detailed morphological evolution of knickpoints over time. The gradients of knickpoint headwalls are sustained and can grow over time as they migrate through headward erosion. This morphology, associated plunge pools, and/or development of enhanced downstream erosion are linked herein to the formation and maintenance of hydraulic jumps. These insights of autogenically driven, temporally high-frequency knickpoints reveal that cut-and-fill cycles with depths of multiple meters can be the norm in submarine systems.

Relating seafloor geomorphology to subsurface architecture: How mass‐transport deposits and knickpoint‐zones build the stratigraphy of the deep‐water Hikurangi Channel

“…The generation of apparent stratigraphic hierarchy by the localized action of different scales of transient seafloor features (the ‘knickpoint migration’ model presented herein; Fig. 17A; see also Guiastrennec‐Faugas et al ., 2021) departs from conventional ‘cut‐and‐fill’ models (Gardner et al ., 2003; Maier et al ., 2012; Fig. 17B).…”

“…Knickpoints migrate upstream by headward incision and downstream deposition. They can either exist as solitary channel‐floor features, or as part of knickpoint‐zones ( sensu Heijnen et al ., 2020; Guiastrennec‐Faugas et al ., 2020, 2021) within which, multiple, closely‐spaced knickpoints collectively form longer reaches of elevated average longitudinal gradient. The formation of knickpoints and knickpoint‐zones may allow deep‐water channels to attain or maintain an idealized ‘equilibrium profile’ (Pirmez et al ., 2000; Kneller, 2003; Guiastrennec‐Faugas et al ., 2020).…”

Channel incision into a submarine landslide on a Carboniferous basin margin, San Juan, Argentina: Evidence for the role of knickpoints

“…Given that these channels are cut into a >200 m thick MTD, then there is probably substantial relief, and commensurate high gradients, on the basin‐facing slope of the landslide (Martínez‐Doñate et al, 2021; Steventon et al, 2019). Flows traversing such steep slopes are known to be associated with upstream migrating knickpoints (Tek et al, 2021), and knickpoint heights can be >10 m in submarine channels (Gales et al, 2019; Guiastrennec‐Faugas et al, 2020, 2021; Heijnen et al, 2020). This suggests that the data are consistent with several discrete phases of knickpoint erosion, separated by sufficient time for lag deposits to accumulate; these lag deposits becoming terrace surfaces following the next incision phase (cf.…”

“…Here, the sedimentological record of these episodic knickpoint periods is restricted to the stepped (terraced) and narrow conduit, the bedload lag deposits stranded on these terrace surface, and the abrupt facies change on top of the lag deposits. This contrasts with studies of modern systems that have also recognised extensive aggradational deposits, including bedforms, across and downstream of knickpoints (Chen et al, 2021;Guiastrennec-Faugas et al, 2021). Such deposits have been identified from flows cutting into previously deposited sand-rich deposits, rather than in cases such as this where the erosion is into mud-rich MTDs.…”

Seismic and Acoustic Monitoring of Submarine Landslides: Ongoing Challenges, Recent Successes and Future Opportunities

“…Technological advances have triggered a recent growth in geophysical monitoring of aspects of submarine landslides including: i) repeated seafloor surveys (at timescales from decades to minutes in some cases) to document elevation changes, evolution of the landslide itself, the effects of landslide runout on the seascape, and subsequent reworking by other marine processes (e.g. Smith et al, 2007;Biscara et al, 2012;Kelner et al, 2016;Mastbergen et al, 2016;Fujiwara et al, 2017;Chaytor et al, 2020;Heijnen et al, 2020;Guiastrennec-Faugas et al, 2021;Normandeau et al, 2021); ii) time-lapse reflection seismic surveys to monitor changes in subsurface conditions (e.g. Blum et al, 2010;Hunt et al, 2021;Roche et al, 2021;Waage et al, 2021); (iii) direct monitoring of turbidity currents (some of which likely initiated from submarine landslides) using moored or vessel-based, active acoustic sensors, such as Acoustic Doppler Current Profilers (ADCPs) and multibeam sonars, that enable measurement of flow velocity and estimation of suspended sediment concentrations (e.g.…”