Modern Slopes

Little Bahama Bank:  A Modern Example

The Bahamian carbonate continental slope is considered a great modern example. It has been extremely well-studied by carbonate sedimentologists from around the world (for example, Alvin (the submersible) made five dives off of Little Bahama Bank in 1971), and recently was the site of the Ocean Drilling Program's Leg 166.

These slopes are almost entirely made up of carbonate material that has been deposited there; as opposed to many ancient systems which are characterized by large contributions from terrigenous material (i.e., debris flows). This is not to say that in the Bahamas carbonate slopes do not experience turbidity currents and debris flows. Rather, these turbidites can be termed "calciturbidites" as a result of their high calcite content.
 
(Coniglis and Dix,1992)  Contour lines showing the topography of the continental slope off of Little Bahama Bank.

Little Bahama Bank is exposed to the open ocean and has a steep upper slope (4º) which is cut by numerous submarine canyons cut by the erosive turbidity currents that travelled down slope. These canyons generally do not extend out on to the lower slope.  The upper slope is composed of periplatform oozes (platform-derived and pelagic muds) that consist mainly of aragonite and high and low Mg-calcite sediment. As the sediments collect on the slope surface, they become cemented.

The lower slope is composed of the material that was eroded from the upper slope (60%) as well as periplatform oozes and pelagic deposits. This portion of the slope is less steep (1-2º). The result is that the entire carbonate slope is concave up and can be considered to be largelymodified by erosion.

The sedimentation accumulation rates in the Bahamas are very variable -- ranging from 1-3 cm/1000 years in the northern Bahamas to 60 cm/1000 years in places such as the Tongue of the Ocean.

The different facies off of Little Bahama Bank are shown below. Here there is a transition from hardground surfaces to oozes on the upper slope to apron facies on the lower slope.
 
A seismic profile illustrates slope failure off of the Little Bahama Bank: The wall collapsed and blocks of material have slid down slope along the erosional surface (Coniglis and Dix, 1992), * Proximal Apron Facies: mud-supported debris flow deposits and coarse grained turbidites; ** Distal Apron Facies: grain-supported debris flow deposits and finer grained turbidites.

Other locations where modern carbonate slopes can be observed include:

   1. Off other islands in the Bahamas (e.g., ODP Leg 166, ODP Leg 101)

   2. Southeast and Southwest Florida

   3. Belize

   4. Jamaica

   5. Grand Cayman

   6. atolls in the Indian and Pacific Ocean

   7. Great Barrier Reef, Australia (e.g., ODP Leg 133)


Click here to view comparison chart between ancient and modern slopes in the chapter on Slope type and Morphology

Click here to examine properties of Ancient Slopes

References and Sources:

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.

Wilson, J.L. carbonate Facies in Geologic History. Springer-Verlag, New York: 1975.

Ancient Slopes


Monday, April 08, 2013
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