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.
Highlights
Turbidity currents are sensitive to changes in orientation and gradient in seafloor topography, and to the nature of lateral confinement
We have primarily focused on siliciclastic depositional systems, but lobes can be deposited from deep-sea canyons and channels in carbonate systems
An increasing number of studies of modern systems suggest that the classic channel-lobe transition zone (CLTZ) of Wynn et al (2002a) is one type in a range of configurations
Summary
Turbidity currents are sensitive to changes in orientation and gradient in seafloor topography, and to the nature (height and form) of lateral confinement. At submarine sites of abrupt changes in lateral confinement, such as at the mouth of submarine channels, or gradient changes, such as at the baseof-slope, flows undergo rapid changes in character (i.e., velocity, stratification, thickness) (e.g., Mutti and Normark, 1987; Mutti and Normark, 1991). These changes profoundly influence patterns of erosion and deposition, and flowtopography interactions are dynamic in time and space. Channels in many examples appear separated from lobe deposits by transitional areas, widely referred to as channel-lobe transition zones (CLTZs; Mutti and Normark, 1987; Mutti and Normark, 1991; Wynn et al, 2002a). Mutti and Normark (1987) defined a CLTZ as “the region that, within any turbidite system, separates well-defined channels or channel-fill from well-defined lobes or lobe facies.” on review of the literature, including recent studies of modern seafloor systems, we propose that CLTZs are one type of a broad spectrum of configurations at channel mouth settings, and consider the likely dominant sedimentary processes and stratigraphic expression of these types
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