(Wilson,
1975)
Carbonate slopes are composed of biological organisms that either rain down through the water column or live on them. The biological organisms that rain through the water column are typically calcareous nanoplankton (coccoliths) and microplankton (foraminifers). Interestingly, calcareous plankton have only been around since the late Mesozoic; therefore there are no pelagic carbonate deposits before this time.
Carbonate slopes are affected by the bottom-dwelling organisms that burrow into them. Different organisms living in different areas along the slope experience very different things...
Factors affecting the type of biology living on the slope include:
Commonly, on the lower slope is composed of bioclastic detritus derived from the upper slope deposits. On the upper slope, communities of whole fossil organisms can be seen, as well as some bioclastic detritus.
Some of the calcareous marine algae that can be seen on the slope (and shelf):
(Wilson,
1975)
codiaceans: a type of algae; Halimeda is an example (click here to see pictures of Halimeda)
oncoids: a type of ooid grain that is larger than 2 mm built of concentric bands which are not perfectly concentric rather they are more lumpy.
an oncoid
What is raining through the water column?
Coccolithophores:
phytoplankton (8-20 micrometers in diameter=nannoplankton) that sercrete
calcareous plates. Today, they are the second most important primary producers
in the world oceans. These make up the chalk deposits at the White Cliffs
of Dover.
Foraminifera:
zooplankton made up of calcium carbonate
Pteropods:
composed of aragonite; most only live in water depths less than 500 m;
especially fond of warmer waters (i.e. tropical places)
Calcareous Ooze: can be made up
of any of the above plankton and are deposited on seafloors that are above
the carbonate compensation depth (CCD). The plankton must become
aggregates in order to fall to the sea floor (this can be accomplished
through copepod fecal pellets).
Fossils on the Carbonate Slope:
Fossils are commonly seen on the bedding planes of the carbonate strata as either body fossils or trace fossils.
Body fossils of macro-organisms in pelagic carbonate deposits are very rare; however, on the shelf, they are very abundant. Therefore, on the continental slope a gradiation from greatest abundance at the upper slope to lowest abundance at the tow of the slope is observed.It may be that the limited amount of oxygen in the bottom waters limits the extent of these creatures.
Trace fossil assemblages that are commonly seen on the slope include: Chondrites, Helminthoida, Paleodictyon, Planolites, Teichichnus, and Zoophycos.
(Scholle
et al., 1983)
Paleodictyon
Bioturbation is a large factor that affects the preservation potential of trace and
body fossils.Heavy bioturbation
results in the destruction of most/all fossils.
Diagenesis of Limestones: affects their porosity, permeabililty, mineralogy, structural strength, and grain size. It can be caused by "pressure solution"-- that is the dissolution of a grain by the pressure of one grain on another. Diagenesis results in the transformation of limestone to dolomite (dolomitization), also aragonite is replaced by calcite.
Hardgrounds: result from the cementation of pelagic sediments by high Mg-calcite (aragonite) and/or glauconite and/or calcium phosphate.Hardgrounds are best formed when the sediment is exposed to the seawater for an extended period of time (i.e., slow sedimentation rate- either from winnowing away of sediments or slow supply rate).Numerous hardgrounds can form in one area as sea level is lowered over time (with the time from the formation of one hardground to the next being thousands of years).
They reduce the porosity of the sediments by as much as 90%.
They are important because they act as the substrate that organisms can burrow into. Many organisms need a hard substrate because they are unable to burrow into soft, muddy, water laden sediments.
The presence/absence of a hard substrate determines to what extent organisms can develop. The sometimes surprising presence of deep hardgrounds results in the presence of bioherms deeper than usually thought.
Cementation: Carbonate sediments are almost always cemented by carbonate minerals, as they do not become very compacted after burial (as do siliclastic sediments). Burrows of organisms can provide areas in which coarse sediments can become trapped and cemented. The cementation process first affects coarser (more permeable) sediments. The cementation cycle is one that begins with an initial cementation, after which organisms burrow into the substrate (which has hardened as a result of the initial cementation), entrapment of sediment by the organisms and hence more cementation of the surface. Also, as a slope steepens, it becomes more easily cemented (a positive feedback).
The cementation of an area may (or may not) continue until a hardground is developed. The amount of cementation decreases downslope because of the decreasing influence of currents.
Evidence of cementation
can be seen in the concentric onion skin-like coatings of the crust, each
about 10 cm thick.
Table showing the modern and ancient carbonate sediment components.
http://www.science.ubc.ca/~geol202/images/sed/carb/pic/carbtab2.gif
Sources:
Oncoid picture: http://xray.geol.uni-erlangen.de/pal/microfac/oncos_g.htm
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.Scholle, P.A., Bebout, D.G., Moore, C.H., (Eds.). Carbonate Depositional Environments. AAPG Memoir 33, Tulsa: 1983.
Stanley, S. Exploring the earth and life through time. W.H. Freeman and Company, New York: 1993.
Wilson, J.L. Carbonate Facies in Geologic History. Springer-Verlag, New York: 1975.
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