Submarine Channel Mouth Settings: Processes, Geomorphology, and Deposits

Abstract

Observations from the modern seafloor that suggest turbidity currents tend to erode as they lose channel-levee confinement, rather than decelerating and depositing their sediment load, has driven investigations into sediment gravity flow behaviour at the mouth of submarine channels. Commonly, channel mouth settings coincide with areas of gradient change and play a vital role in the transfer of sediment through deep-water systems. Channel mouth settings are widely referred to as the submarine channel-lobe transition zone (CLTZ) where well-defined channel-levees are separated from well-defined lobes, and are associated with an assemblage of erosional and depositional bedforms (e.g., scours and scour fields, sediment waves, incipient channels). Motivated by recently published datasets, we reviewed modern seafloor studies, which suggest that a wide range of channel mouth configurations exist. These include traditional CLTZs, plunge pools, and distinctive long and flared tracts between channels and lobes, which we recognise with the new term channel mouth expansion zones (CMEZs). In order to understand the morphodynamic differences between types of channel mouth settings, we review insights from physical experiments that have focussed on understanding changes in process behaviour as flows exit channels. We integrate field observations and numerical modelling that offer insight into flow behaviours in channel mouth settings. From this analysis, we propose four types of channel mouth setting: 1) supercritical CMEZs on slopes; 2) plunge pools at steep slope breaks with high incoming supercritical Froude numbers; 3) CLTZs with arrays of hydraulic jumps at slope breaks with incoming supercritical Froude numbers closer to unity; and, 4) subcritical CLTZs associated with slope breaks and/or flow expansion. Identification of the stratigraphic record of channel mouth settings is complicated by the propagation, and avulsion, of channels. Nonetheless, recent studies from ancient outcrop and subsurface systems have highlighted the dynamic evolution of interpreted CLTZs, which range from composite erosion surfaces, to tens of metres thick stratigraphic records. We propose that some examples be reconsidered as exhumed CMEZs.

Exceptional preservation of three-dimensional dunes on an ancient deep-marine seafloor: implications for sedimentary processes and depositional environments

“…Arnott, 2011;Tilston et al 2015), may be the reason for the lack of dunes in ancient deep-water successions, as dunebearing packages will be frequently cannibalised by avulsion or propagation of their associated channels (e.g. Hodgson et al 2022), or simply not recognised at outcrop due to their relatively low thicknesses and affinity with weathered fine-grained sediments. The preservation potential of these deposits in the Aínsa Basin may have been favoured by enhanced lateral migration of channels adjacent to the thrust front (e.g.…”

“…The preservation potential of these deposits in the Aínsa Basin may have been favoured by enhanced lateral migration of channels adjacent to the thrust front (e.g. Bayliss and Pickering, 2015), resulting in channel mouths and overbanks being quickly abandoned and not cannibalised (Hodgson et al 2022), high aggradation rates (Pemberton et al 2016;Hodgson et al 2022), early cementation aided by intense bioturbation and high bioclastic (and hence carbonate) content, or a combination of these factors. This is a non-peer reviewed preprint submitted to EarthArXiv 28…”

The response of high density turbidity currents and their deposits to an abrupt channel termination at a slope break: Implications for channel–lobe transition zones

“…This switch to deposition takes place via a channel mouth that often coincides with areas in which TCs encounter topographic slope breaks (SB) (Hodgson et al ., 2022), such as the mouth of the Rhone Canyon, where there is a slope reduction of 0.3° (Wynn et al ., 2002). The recent study by Hodgson et al .…”

“…The recent study by Hodgson et al . (2022) identifies four key channel mouth settings: (i) channel mouth expansion zones dominated by supercritical turbidity currents; (ii) plunge pools that mark the base of steep slopes and are also created by supercritical turbidity currents; (iii) channel–lobe transition zones at shallower slope breaks characterized by hydraulic jump arrays within turbidity currents that have Froude numbers close to unity; and (iv) subcritical channel–lobe transition zones related to slope breaks and/or flow expansion.…”

“…This switch to deposition takes place via a channel mouth that often coincides with areas in which TCs encounter topographic slope breaks (SB) (Hodgson et al, 2022), such as the mouth of the Rhone Canyon, where there is a slope reduction of 0.3° (Wynn et al, 2002). The recent study by Hodgson et al (2022) identifies four key channel mouth settings: (i) channel mouth expansion zones dominated by supercritical turbidity currents; (ii) plunge pools that mark the base of steep slopes and are also created by supercritical turbidity currents; (iii) channel-lobe transition zones at shallower slope breaks characterized by hydraulic jump arrays within turbidity currents that have Froude numbers close to unity; and (iv) subcritical channel-lobe transition zones related to slope breaks and/or flow expansion.…”