virtual-geology.info

Introduction

This page is arranged roughly in stratigraphical order, and will be modified frequently. University of Derby students will find additional information and diagrams in the handouts which form part of the Sedimentology module.

Reefs and Buildups through Time

Carbonate-producing organisms have varied through geological time, so modern reefs are different from those formed in the geological past.

• Stratigraphic column showing reefs and reef builders through Phanerozoic time

There were periods of Phanerozoic time when the construction of reefs was important, notably the Ordovician, Silurian to Devonian, Mesozoic and Tertiary. At other times, reef formers were less important as producers of carbonate sediment.

Archaean to Recent

Cyanobacteria - the earliest reef builders (>3000 Ma), still around at the present time.

During the Precambrian (>80% of geological time), the only reef^*-building organisms were the cyanobacteria, a primitive group of photosynthesizing organisms which form cyanobacterial mats. Remains of these are found back to >3000 Ma. The cyanobacteria perform various different activities in sedimentary environments. We have already seen one example - bio-erosion and micritization of carbonate material by endolithic cyanobacteria. Other types of cyanobacteria (wrongly called 'blue-green algae') are the main component of microbial mats (often wrongly called algal mats).

[^*Not all carbonate workers would regard structures built by non-skeletal organisms as reefs]

Lyngbya. Filamentous cyanobacteria. Images courtesy of Cyanosite, Purdue University, Indiana.

Filamentous cyanobacteria trap fine-grained carbonate (and sometimes non-carbonate, especially aeolian dust) sediment. They form gelatinous to leathery flexible sheets containing sticky organic matter (mucillage), to which sediment sticks. When the mat becomes covered in sediment, it grows another layer of filaments through and over the sediment, so it can continue to photosynthesize. Then another layer of sediment is trapped, building up in layers which are alternately organic-rich and sediment-rich.

This combination of cyanobacterial mat and sediment is a stromatolite. Microbial mats allow fine-grained sediments to be deposited in environments whose energy would normally be too high for such deposition. The photosynthesis of the cyanobacteria removes CO₂ from waters within the mat, promoting the precipitation of CaCO₃, often as early cement, which causes the mat to harden.

Modern stromatolites are found mainly in warm climates and very shallow water. They occur in shallow subtidal, intertidal environments, and may extend into the supratidal environment where the climate is humid. They also occur in freshwater. See:

• stromatolite environments

Stromatolites show a variety of forms, related to environmental conditions, such as energy level and rate of sedimentation. This is a simple example of more or less parallel-laminated stromatolites. More complex forms develop when energy and sedimentation rate are high. (Lower Jurassic, S France)

Internally, stromatolites show distinctive textures:

Desiccation features are common in stromatolites, particularly those formed in the intertidal or supratidal environment. They often contain irregular cavities (fenestrae) parallel to the lamination, a form of primary porosity resulting from shrinkage and decay of the organic material of the mat.

	Some stromatolites are mobile, rather than fixed - oncolites. Cyanobacteria grow around a nucleus, such as a sedimentary grain, shell or a fragment of detached microbial mat. Laminations, as in stromatolites, tend to be highly irregular.
	These examples are from non-marine Upper Cretaceous strata in southern France.

• Kingdom Monera: The Cyanobacteria
• Cyanobacteria
• Introduction to the Cyanobacteria
• Stromatolite images
• Hamelin Pool Marine Nature Reserve, Shark Bay, W Australia
• Growing stromatolites in the lab - images

Let us look now at examples of other organisms which have been important reef formers in the geological past.

Cambrian

• Archaeocyathid reefs
• Early Cambrian reef diagram
• Late Cambrian reef diagram

Ordovician

Tabulate & rugose corals became common. Stromatoporoids appeared.

• Early Ordovician reef diagram

Silurian

The main reef builders in the Silurian & Devonian periods were: tabulate & rugose corals; stromatoporoids.

Favosites, a Silurian tabulate reef former from Wenlock Edge, England.

• The Virtual Silurian Reef. Milwaukee Public Museum

Devonian

	Cross section of massive reef rock in a quarry in the Meuse valley, S Belgium. The sheet-like colonies are mostly stromatoporoids, with some tabulate corals (lighter coloured lens, at centre). Compare with this Late Devonian reef diagram
	In this polished 'marble' slab, the red material is part of a micrite mud mound, and the white areas were cavities with planar floors and irregular roofs (possibly formed by collapsed sponges), later filled with calcite cement. Stromatactis from the Middle Devonian of S France. For more information, see: Frasnian micrite mud mounds from Belgium. Very well illustrated site.
The "Devonian Great Barrier Reef", Canning Basin, W Australia Late Devonian back reef diagram

Carboniferous

• Carboniferous stromatolite image
• Early Carboniferous reef diagram
• Early Carboniferous mud mounds

A Carboniferous reef-building rugose coral. Rugose & tabulate corals became extinct in the Permian.

Permian

Sponges & calcareous algae dominated.

• The Permian Reef Complex (Delaware Basin) of West Texas. This 'virtual field trip' complements the Open University video on the Capitan Reef.
• A more detailed account of this case study: An Introduction and Virtual Geologic Field Trip to the Permian Reef Complex, Guadalupe and Delaware Mountains, New Mexico-West Texas
• Bryozoan reef diagram
• Late Permian bryozoan reef
• Late Permian sponge reef

Triassic

Scleractinian corals became important in the Triassic.

Jurassic

• Jurassic Reef Park, University of Stuttgart
• Sponge reefs
• Jurassic coral patch reef
• Jurassic oyster reef

Cretaceous

• Rudist buildup diagrams
• Permian rudist buildups in Texas

Rudist bivalves have a similar morphology to some corals, but don't branch or bud, and there are no encrusters filling the gaps, because rudists are surrounded by sediment. They were supported by leaning on each other, and by the sediment. They grew rapidly.

	Upper Cretaceous rudist 'reef' from S France. The rudist bivalves, in growth position, indicate the way-up of the bed. Between the fossils is sandstone. (Abbaye de Fontfroide, Aude)
	Cut section through part of a rudist 'reef' from the same locality as above. Note geopetal infills in some of the rudists.

Tertiary

Scleractinian corals continued as a major reef former in Tertiary times. One well-studied example is the Miocene reefs of Mallorca.

Cliff sections in Miocene reefs at Cap Blanc, Mallorca. These superb exposures have enabled detailed analysis of reef development by Prof Luis Pomar and co-workers.

Quaternary

	Cross-section through a brain coral in the Pleistocene Key Largo Limestone, Windley Key Quarry, Florida Keys
	Modern reef at Key Largo Dry Rocks, Florida Keys. Note the large, massive scleractinian coral colonies

Modern reefs

• Block diagram: Indo Pacific modern coral reef
• Coral algal reefs, Bermuda
• US coral reefs

Modern reef images

• Coral Kingdom photo library
• Hawaii

This page is maintained by . Last updated 26 January, 2007 15:06