Background

Background:-

What are submarine channels?
Why are submarine channels important?

Submarine channels are the principal conduits for the transport of sediments from shallow-marine/shelf to the deep-marine (or deep lacustrine) environments.
Submarine channels have been documented from many ancient successions world-wide, the present-day seafloor and areas of subsurface hydrocarbon exploration and development.
The general distribution of aspect ratios (width:depth ratios) in submarine channels is similar in both modern and ancient settings, and is typically in the range 10:1 to 100:1 (after Clark & Pickering 1996).

The variation of channel sinuosity with increasing gradient is an important relationship which has been observed in rivers and from flume tank experiment. Studies of submarine channel sinuosity and channel slope show a comparable correlation to that seen in fluvial systems (see Clark, Kenyon & Pickering 1992). In this paper, we did not derive bell-shaped fitted curves from the submarine channel data, but rather showed a "best-fit" curve for each selected submarine channel which was extrapolated from fluvial and experimental-flume data and, therefore, assumed comparability. Whatever the limitations of this method, it produced a sensible classification of high, moderate and low-sinuosity submarine channels.

The comparison between fluvial and submarine channels appears robust for many morphological features in both types of channel systems. However, there are important reasons to expect significant differences (see Clark, Kenyon & Pickering 1992): (a) Maximum flow depths in rivers only exceed bankfull depths during flood conditions, whereas it is common for turbidity currents to exceed the height of the channel levees; (b) Many fluvial channels experience continuous flow conditions, but submarine channels are more analogous to flash-flood or ephemeral stream channels; (c) In submarine channels, it seems reasonable to predict that flow-stripping (sensu Piper & Normark 1983) at channel bends is relatively common unlike in fluvial environments; (d) In rivers there is no entrainment of the overlying ambient fluid (air) into the flow, and (e) In submarine channels, the Coriolis effect influences the location and height of levees, and the lateral migration/stacking pattern of channels. To date, there is little unambiguous evidence for lateral accretion processes in submarine channels being important, but rather sequential "jumps" in the location of a channel course (lateral offset stacking - see below). Ancient outcrops of submarine channels rarely show candidate lateral accretion surfaces. Caution, however, should be exercised in totally dismissing lateral accretion as a process in submarine channels, because there is very limited 3D high-rersolution data from modern submraine channels.

Channel stacking patterns are likely to be related to sinuosity, rate of sediment accumulation, sand/mud ratio of sediment supplied (net/gross ratio), and the degree of channel confinement (as is shown in the figure below, after Clark & Pickering 1996).

In summary, sites of preferential sand accumulation in submarine channels can be listed as follows (after Clark & Pickering 1995, 1996): (1) Channel bends associated with flow-stripping; (2) Channel confluences; (3) Point-bars associated with high-sinuosity lateral accretion; (4) Channel benches and terraces; (5) Channel thalwegs; (6) Intra-channel hydraulic-jump sites, e.g. transverse channel benches caused by deep erosional features and growth faults.

Submarine channels are important because they are:

Major hydrocarbon reservoirs (e.g. Gulf of Mexico, Brazilian margin, Nigerian margin, North Sea basin, West-of-Shetland, North Slope off Alaska, Far East deep-water slopes/basins);

Natural laboratories in which to study Earth surface processes (e.g. turbidity currents, debris flows, deep-water focussing of tidal currents);

Potentially useful in military manoeuvres. They could provide pathways along which submarines might travel with greater stealth.