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The continental
slope is the steepest part of the ocean floor that exists
between the nearly horizontal continental
shelf and the deeper ocean floor called the shelf-slope
break. The continental slope is narrow and has an average angle of
4°-6° in passive margins, the slope will gently slope upwards into
the ocean floor and form what is called the continental rise. If
the margin is tectonically active, then the slope will drop off into the subducting
trench. Much sedimentation occurs in this area. Sediments deposited
are mainly done so by gravity transport and turbidity currents. These
processes cause distinguishing sequences that can easily be identified
after knowing certain characteristics of these sediments.
Because of the steep slope of the continental slope,
gravity transportation of sediments is quite common. Osoliths,
olistromes
and tubidites occur in the occurrence of a gravity, or debris flow. Osoliths are
large, exotic slide blocks that have slumped, slid or flowed down the slope. The breccias that occur in these slumps are called olistromes. In the geological record, a slump
can look like folds and because of the olistromes
looking sheared, some may mistake these areas for tectonically deformed
areas. However, the olistromes tend
to be less deformed than tectonic breccias or mélanges. Also
tectonic breccias tend to have secondary shearing. To further distinguish osoliths
from tectonically deformed areas, look at the sediments around the olistromes. They should be bound by deep-sea sediments this is because they are sedimentary
rocks.
The continental slope has another kind of distinguishing sediments. They consist of hemapelagic muds that fall out of suspension from the water
column. Contour currents can rework these muds. The finely
laminated deposits that result form such an occurrence is called contourites. These sediments can be hard to distinguish.
Turbidity currents are called the most important process of sediment
transport. This current is able to carry larger particles because
of the higher velocities. Turbidity currents are the reason that
larger particles are carried to the deep parts of the ocean.Large particles
are not common in this are of the ocean. Turbidity currents are caused
by some catastrophic event (i.e. a debris flow). The resulting sequence
is called a turbidite.The
predictable sequence that exists in a turbidite
is known as the Bouma sequence.The
sequence is characterized by a massive graded bed that has a fining upward
sequence. The bottom possesses the coarsest material, which is able to
settle out of the high velocity currents.The coarse material is overlain
by sand that is laminated, but still in high velocities. As velocities
slow down sand and silt begin overlaying the laminated sand. This
unit often shows ripple marks. The turbidity eventually slows down
to where pelagic mud can fall out of suspension and cover the Bouma
sequence.
There are other distinguishing characteristics of the continental slope
when interpreting the geologic record.These can include
flute casts, groove casts, and some pelagic organisms
that will occur as fossils.Flute casts and groove casts are caused by scouring
currents that can exist on the slope.
These are just some of the distinguishing characteristics
of the continental slope, what rally dominates the slope are hamepelagic
sediments that have
submarinechannels and
fans
that interrupt them.
Fans have a coarsening upward sequence. Sedimentation in slope fans is dominated by sea level. When the sea
level is low the sediments can accumulate at heads of submarine fans.
The typical geometry of slope
and
rise are thick wedges that may be up the thousands of meters thick, kilometers
wide and extend kilometers across a basin.
Reference Prothero, Donald R. Interpreting the Stratigraphic Record. W. H. Freeman and Company,
1990.
photo: "Erosion by Biological Activity
in Two New England Submarine Canyons" by W.P. Dillon and H.B.
Zimmerman and published in the Journal
of Sedimentary Petrology, Vol 40. No. 2, p. 542-547, June, 1970.
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