Biology

 

Biological Factor

The majority of carbonate sediments are produced in response to biologically or biochemically mediation.

There are four main types of carbonate sediment particles:

  • Precipitates: formed by direct or biologically mediated precipitation of calcium carbonate; examples include ooids and lime muds
  • Bioclasts: the calcareous shells, tests, spicules and other body support structures
  • Physico-chemical grains: agglutinated or cemented carbonate, carbonate grains, or diagenetically altered materials
  • Lithoclasts: fragments of consolidated, hardened or lithified sediments

The majority of carbonate sediments are the skeletal remains of carbonate secreting organism and precipitates surrounding phototrophic organisms. These later include most photosynthetic marine organisms that range from microbes, cyanobacteria and algae to corals and even giant clams. Most of these organisms are limited to near surface settings within the photic zone. Modern to ancient carbonate skeletal deposits can be compared through time even though marine species change their form with evolution, but the physical organization within these carbonate particles does not. Two expressions of skeletal carbonates include:

  • Gross external shape as with corals and snail shells.
  • Differences in internal fabric an organization as for example in trilobites and clams. In the latter cases this internal fabric is formed by multiple segments held together by some other soft tissue.

Once these carbonate particles are formed and accumulate other biological organisms, such as sea grasses and microbes, act as sediment traps and stabilizers. In this way the entire carbonate factory is controlled by and evolves with the biology of its own system.

There are three main faunal assemblages associated with the carbonate factory.

  • Benthic foraminifera, molluscs, barnacles, brozoa and calcareous red algae. This assemblage lives in the cooler waters below 18 degrees Celcius.
  • Hermatypic corals, calcareous green algae and foramol constituents. This is the warm water assemblage.
  • Calcareous green algae. These algae dominate the high salinity assemblages.
Foramol2. Chlorozoan3. Chloralgal

Index to carbonate shelf sediments

Shallow Shelf Carbonates Carbonate Factory Evolution
Lag time Antecedent Topography Biology
Climatic Zone Siliclastic Influx Temperature and Salinity
Sea Level Clastic Input Tectonism
Platform Morphology Unrimmed Shelves Rimmed Shelves
Banks Stratigraphic Succession Water Depth and Turbidity
Subsidence Lithofacies Circulation
Carbonate Growth Potential Eustatic Response Questions

References
Wednesday, April 03, 2013
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