Modern Environments of Carbonate Sedimentation

From a sedimentological point of view a reef may be defined as a laterally restricted body of carbonate rock whose composition and relationship with the surrounding sediments suggest that the bulk of its biota were bound together as a framework during deposition, maintaining and developing a positive topographic structure on the sea bottom.

Modern reef structures are dominated by hermatypic corals and calcareous red algae (Fig. 5.39), but the biotic composition of the reef structures through the Phanerozoic has been fairly variable (Fig. 5.40). Generally, all these different reef types show similar facies patterns to modern coralgal reefs. Recent reef formation, and the shape of the reef complexes, is essentially a function of the ecological environments of the reef-building organisms:

1. Reef-building corals require warm surface water and their distribution today is therefore limited to the lower latitudes. However, palaeogeographical reconstructions must take into account that surface water temperature is not a simple function of latitude. This is illustrated by the coasts of West Africa

and along the western side of the American continent. Where there is upwelling of cold water the surface layers are in most cases too cold for reefs to form, even near the equator. Off the coast of East Africa and in the Caribbean, on the other hand, the water is warm because of prevailing onshore winds, and coral reefs are abundant.

2. The hermatypic corals which build reefs live in symbiosis with algae and require sunlight. The reefs are therefore sensitive to changes in sea level since reef organisms tolerate neither exposure nor “drowning” below the photic zone. The Holocene transgression about 10,000 years ago raised sea level about 100 m in a few thousand years, but most reefs managed to grow quickly enough to keep pace with the rising sea level.

3. If sea level rises faster than the coral reef can grow, the reef may drop below the photic zone and “drown”, but this is very rare because they can gain height rapidly.

4. Reefs provide a favourable ecological environment for animals which are not part of the reef structure itself, but which live on other organisms.

5. Many organisms in coral reefs live by filtering water to trap planktonic organisms and organic material. If the water contains too much siliciclastic mud, the clay minerals can choke the filtering organs so that the organisms die. Corals are particularly sensitive to the clay content in the water and can only live in clear water. The addition of clay, for example from a delta, will kill a coral reef. Pollution will have the same effect.

6. Clear sea water, however, is usually very poor in nutrients, so the organisms in a reef are dependent on good water circulation to obtain enough food. Consequently, reefs tend to grow on the edge of ocean basins, or as structures projecting high up from deeper waters. In this way, high water temperature is combined with low mud content and good water circulation.