Clastic Slopes

Clastic Slope Systems
Welcome to our Clastic Slope Systems page. We hope to provide you with an overview of the processes involved in the deposition and sediment transport for this environment. This page was originally created as a project for a USC Marine Sediments (Fall 2000) class project.     
turbidites in Action
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.

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,
Monday, April 08, 2013
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