Accurate interpretations of facies in ancient reefs requires both proper “classification” of the reef and an understanding of the sedimentologie and biologic processes active during the formation of the reef. Study of modern reefs provides evidence for the processes affecting facies in reef complexes with a rigid organic framework and steep fore-reef wall, but generally reveals only the incipient products of these processes because most Holocene reefs have been growing for only a few thousand years following the Holocene transgression. Only where underlying topography approximates the profile of a mature reef complex can well developed examples of the sedimentologie facies of a mature reef complex be found in modern reefs. Study of these pseudo-mature reef complexes reveals a sequence of sedimentologie facies (from basin toward land) consisting of: I. distal talus, 2. proximal talus, 3. reef-slope, 4. reef framework, 5. reef-crest, 6. reef-flat, and 7. back-reef sand. Knowledge of the characteristics and distribution of these facies should facilitate facies interpretation in comparable ancient reef complexes.
Reef framework forms only a few percent of the volume of a mature reef complex, whereas the vast majority of the complex consists of debris in fore-reef and back-reef facies; the debris being derived largely from the framework. This has particular significance in hydrocarbon exploration because most wells drilled toward the top of a seismicly defined ancient reef complex would penetrate only the back-reef carbonate sand facies. However, this is as it should be because the reef framework facies generally has little preserved primary porosity due to sediment infilling of cavities and extensive marine cementation.
The facies of modern reef complexes discussed here exist due to the presence of a significant marginal (as opposed to centrally located) shallow water, wave resistant, rigid organic framework composed largely of scler-actinian corals. Similar facies may be expected in those fossil reef complexes with a more or less comparable organic framework such as the Devonian stromatoporoid/tabulate coral reefs, the Permian algal/sponge/marine cement reefs, and the Neogene coral reefs. However, different facies must occur in ancient reefs that lacked rigid organic frameworks such as the Paleozoic bryozoan and/or crinoid mounds, Late Paleozoic phylloid algal mounds, Cretaceous rudistid banks, and early Tertiary Nummulites banks.