62 Origin and Morphology of Ocean Margins
Canyons and various other types of slope valleys can be extremely abundant in
places, such that the slope is dissected like the remnants of a mesa on land. Nor do
those canyons necessarily run straight downhill; they may be cut at an angle to the
slope. In other places, submarine canyons seem to be absent, presumably by reason
of low sediment supply (insufficient for producing downhill currents) or because of
gentle slopes, or both. The best-known canyons are associated with the mouths of
large rivers - the Congo, Indus, Ganges and the Hudson. The fan valleys which
continue these canyons down slope are bordered by levees (Fig. 2.17).
A large number of hypotheses have been put forward over the years to account for
the origin of submarine canyons. In fact, different types of canyons must have different origins. For example, it has been demonstrated through deep-sea drilling, that the
Mediterranean became isolated from the world ocean some 5 to 6 million years ago
and dried up during periods of evaporation. At those times, deep canyons could have
been carved by the familiar action of rain and run-off, into the margins of the Mediterranean. Indeed, the true floor of the Nile Valley is very deep, supporting the idea
of canyon cutting curing desiccation. At present the river runs on top of a thick pile
of sediment, which has since filled the canyon. However, we can hardly invoke such
drastic falls of sea level everywhere in the world ocean. Thus, there must be a way to
make submarine canyons by cutting them under water. The fact that so many canyons
are off river valleys suggests a mechanism: some sort of submarine river flowing on
the sea floor. This submarine flow cannot be an extension of the river entering the
sea: its water is fresh and therefore less dense than seawater. River water floats on
seawater. But water with a high mud content is heavy enough to flow downhill on the
ocean floor.
One way to stir mud into the water is to have slides start in the soft sediments
deposited off river mouths. Large amounts of mud are brough down the river during
floods, and such mud is rather unstable. Hurricanes and wave action may produce
large amounts of heavy, muddy water through stirring up of the sediment. Earthquakes may be agents for starting mudslides which tum into muddy downhill flow, or
turbidity currents.
During glacial periods, when sea lev en was greatly lowered by the buildup of ice
caps, the exposed shelves could not act as mud traps for the rich sediment load
coming from land. Also, wave attack must have been strong and its effects may have
reached much greater depths than today. Storms and stonn waves were probably
more frequent then, so that sediment on the outer shelf and on the upper slope could
be resuspended periodically, providing for mud-laden, heavy water bodies, which
could then move downslope and initiate powerful turbidity currents.
The realization that such currents play an extremely important role in present-day
marine processes as well as in the geologic record, came only in the 1950's, largely
through the work of Ph, H. Kuenen (Fig. 2.15a).
Kuenen established, by experiment, that such heavy downhill flowing currents can
exist in nature and that they would deposit layers which show the grading familiar
from hitherto unexplained sediment-series (f7ysch) in the Alps and other mountain
ranges (Fig. 2.15b).
Canyons and various other types of slope valleys can be extremely abundant in
places, such that the slope is dissected like the remnants of a mesa on land. Nor do
those canyons necessarily run straight downhill; they may be cut at an angle to the
slope. In other places, submarine canyons seem to be absent, presumably by reason
of low sediment supply (insufficient for producing downhill currents) or because of
gentle slopes, or both. The best-known canyons are associated with the mouths of
large rivers - the Congo, Indus, Ganges and the Hudson. The fan valleys which
continue these canyons down slope are bordered by levees (Fig. 2.17).
A large number of hypotheses have been put forward over the years to account for
the origin of submarine canyons. In fact, different types of canyons must have different origins. For example, it has been demonstrated through deep-sea drilling, that the
Mediterranean became isolated from the world ocean some 5 to 6 million years ago
and dried up during periods of evaporation. At those times, deep canyons could have
been carved by the familiar action of rain and run-off, into the margins of the Mediterranean. Indeed, the true floor of the Nile Valley is very deep, supporting the idea
of canyon cutting curing desiccation. At present the river runs on top of a thick pile
of sediment, which has since filled the canyon. However, we can hardly invoke such
drastic falls of sea level everywhere in the world ocean. Thus, there must be a way to
make submarine canyons by cutting them under water. The fact that so many canyons
are off river valleys suggests a mechanism: some sort of submarine river flowing on
the sea floor. This submarine flow cannot be an extension of the river entering the
sea: its water is fresh and therefore less dense than seawater. River water floats on
seawater. But water with a high mud content is heavy enough to flow downhill on the
ocean floor.
One way to stir mud into the water is to have slides start in the soft sediments
deposited off river mouths. Large amounts of mud are brough down the river during
floods, and such mud is rather unstable. Hurricanes and wave action may produce
large amounts of heavy, muddy water through stirring up of the sediment. Earthquakes may be agents for starting mudslides which tum into muddy downhill flow, or
turbidity currents.
During glacial periods, when sea lev en was greatly lowered by the buildup of ice
caps, the exposed shelves could not act as mud traps for the rich sediment load
coming from land. Also, wave attack must have been strong and its effects may have
reached much greater depths than today. Storms and stonn waves were probably
more frequent then, so that sediment on the outer shelf and on the upper slope could
be resuspended periodically, providing for mud-laden, heavy water bodies, which
could then move downslope and initiate powerful turbidity currents.
The realization that such currents play an extremely important role in present-day
marine processes as well as in the geologic record, came only in the 1950's, largely
through the work of Ph, H. Kuenen (Fig. 2.15a).
Kuenen established, by experiment, that such heavy downhill flowing currents can
exist in nature and that they would deposit layers which show the grading familiar
from hitherto unexplained sediment-series (f7ysch) in the Alps and other mountain
ranges (Fig. 2.15b).