There are several ways sediment is transported to and accumulates on carbonate slopes, including:
1)Suspension
settling:
Process in which fine-grained, platform-derived sediments are transported
off the shallow water platform by tidal flux, storms and oceanic currents
as sediment-sea-water mixtures.When
these plumes of material dissipate, this fine-grained material settles
to the sea floor.
Recognition of suspension settling on ancient slopes may be difficult.In places where carbonate is finely laminated, suspension settling may have occurred in an oxygen minimum zone.Ancient slope sediments exhibiting evidence of bioturbation, primary sedimentary laminations, and nodular bedding may indicate an oxygenated slope.
2)Gravity flows: These processes are high-energy, essentially instantaneous events, which transport sediment downslope due to the force of gravity.There are several types of gravity flow:
A)Turbidities:This type of gravity flow originates from locally stirred-up sediment gives water a density greater than the surrounding water.These flows rapidly transport suspended sediment down-slope to the lower slope or basin.They are characterized by graded bedding, moderate sorting and well-developed primary structures such as laminations; these features aid in recognizing turbidites in ancient slope deposits.
Click
here to see an animation of a turbidity current.
B)Debris
Flows: These
flows consist of a mud and water matrix with a combined strength and buoyancy
which allows the flow to carry clasts that are denser then the flow itself
and can travel up to 100 km away from there source.The
deposit that results is sheet-like.Common
features include lenticular beds that gradually pinch out, poor sorting,
lack of stratification, random clast arrangement.Clasts
are rounded to angular, and range in size from gravel to boulders and set
in a matrix, which can vary from fine-grained carbonate to terrigenous
mud.
3).Talus:Talus
forms down-slope widening conical accumulations along the top of the slope
below steep escarpments.Talus blocks
probably tumble into place.Merging
of adjacent talus cones forms a distinctive but narrow facies belt.Identifying
this facies in ancient slope deposits can offer information as to slope
gradient and orientation.
3)Contourite
Facies:
These deposits result from currents flowing parallel to the slope.They
mostly consist of platform sands and gravels, and are characterized by
a high degree of sorting, evenly spaced ripples, lack of mud, and sharp
contacts.Contourites can be used
to identify the orientation of ancient slopes since their origin is a flow
direction parallel to the slope.
The following diagram shows some ways sediment is transported to the slope:
(Coniglio and
Dix, 1992)
The
following diagram shows the ideal stratigraphy that results in the different
depositional facies outlined above:
There
are also several ways and causes for slope erosion and failure.Some
causes of slope failure include:
1)sediment
overloading 2)seismic
activity 3)excessive
pore pressure Some
of the ways in which slopes fail are:
1)Slide:A
slide is the movement of a rigid, internally undeformed mass along a discrete
shear surface.If internal deformation
occurs during the journey down-slope, the sliding mass may turn into a
debris flow.
a)rotational
slump:
curved shear surface
b)translational
glide:planar
shear surface
The
distinction between these surfaces is usually not recognizable in ancient
sequences.
Line drawing of a large
slide at Privitt Place.
2)Slide
scars: A trough or concave depression recognized on modern and ancientslopes
by the abrupt termination of older strata and the anomalous thickening
of infilling younger units.
3)Downslope
Creep: This process results from bedding parallel translation along
a well-defined surface lacking a basal shear plane.This
is a long-term process most common to slopes within an unlithified sediment
pile.
Sources:
Bates,
R.L., Jackson, J.A. Dictionary of Geological Terms. Anchor Books, New York : 1984.
Brown,
A.A., Loucks, R.G. Influence of Sediment Type and Depositional
Processes on Stratal Patterns in the Permian Basin-Margin Lamar Limestone,
McKittrick Canyon, Texas in Carbonate Sequence Stratigraphy:Recent
Developments and Applications. Ed. Loucks, R.G. and Sarg, J.F. AAPG Memoir 57, Tulsa: 1993. p.145.
Coniglio,
M., Dix, G.R. Carbonate Slopes in Facies Models: Response
to Sea Level Change. Ed. R.G. Walker and N. P. James. Geological
Association of Canada, St.Johns: 1992. pp 349-373.
Climate...Sea
Level...Oceanographic Setting...Sediment
Supply and Sedimentary Structures... Sediment
Transport and Erosion....Slope type
and Morphology... Biology...Tectonics...Modern....Ancient...Question
Set...Links
(Brown
and Loucks, 1993)
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