Summary:
The scope of this diploma thesis is to collect and emphasize key issues concerning the studies of reefs and their rocks (reef limestones).
Reefs are biologically constructed reliefs standing out from surrounding seafloor. They occur throughout Earth's history, beginning from the Archean until today. Modern reefs, as well as ancient ones, are located mainly in shallow marine environments, but also at greater depths, on slope to basin. Even though they were previously believed to be confined at tropic to subtropic warm waters, today it is acknowledged that they form in temperate and cold waters and extend up to mid and high latitudes.
Organisms contribute to the development of reefs in different ways, while also having various roles (reef-builders, binders, users). Main modern reef-builders range from corals, to algae, to microbes. Other organisms within the reef environment, such as bivalves, sponges and worms, make these ecosystems quite complex, also produce carbonate mud and destroy the reef surface by bioerosion (borings). Overall microbes, calcareous algae, colonial metazoans, and synsedimentary carbonate cements within internal cavities of the skeletal framework, make up principal structural elements of reefs.
The above-mentioned constituents give rise to a wide spectrum of reef structures and types, the end-members being: microbial reefs, (carbonate) mud mounds, and skeletal reefs. Generally, these appear and prevail sequentially in this order in the geological record. However, their stratigraphic and geographic distribution is rather complex, especially the growth of organisms is influenced by factors including sunlight, depth, temperature, salinity, turbidity, nutrients and exogenetic controls (climate, antecedent topography, relative sea-level fluctuation, ocean chemistry).
Consequently, reef limestones differ from other carbonate rocks, their formation being linked to an interplay of biological, physical and chemical processes. As such, they are responsible for their unique sedimentation and facies patterns, and their specific grouping within the textural classifications of carbonate rocks, mainly based on organic bounding during deposition. Additionally, various post-depositional diagenetic processes alter their primary texture; this, however, may enable them to become particularly productive hydrocarbon reservoirs.
Keywords:
reef types, control factors, reef rocks, lithofacies, diagenesis
