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.

Most photos can be enlarged by clicking on them.

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 (SEPM Strata)

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 CO ₂ from waters within the mat, promoting the precipitation of CaCO ₃ , 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 parallel- to wavy-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. The cylindrical oncolites may have formed around plant stems.

• The Cyanobacteria
• Introduction to the Cyanobacteria
• Microbial Mat and Stromatolite images (NASA)
• Shark Bay, W Australia
• Stromatolites - the role of microbes

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 reconstruction
• Late Cambrian reef reconstruction

Ordovician

Tabulate & rugose corals became common. Stromatoporoids appeared.

• Early Ordovician reef reconstruction

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 reconstruction
	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 reconstruction

Carboniferous

• Early Carboniferous reef reconstruction
• Early Carboniferous mud mounds

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

Permian

Reefs at this time were dominated by sponges, calcareous algae and bryozoa.

Permian reefs at South Elmsall, South Yorkshire, UK. The lower part of the face is in bedded and cross-bedded bioclastic limestones, overlain by discontiuous massive bryozoan reefs, then by a thick development of dome-shaped stromatolites.

• 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, and continued as important reef builders to the present time. Triassic reef from Austria.

Jurassic

Coral limestone with Isastrea sp. (colonial scleractinian coral). White Limestone Formation, Middle Jruassic, Bathonian Stage. Gibraltar Quarry, Oxfordshire. Scale in cm

Middle Jurassic sponge reefs, St Aubin-sur-Mer, Normandy.

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

Cretaceous

• Rudist buildup diagrams

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.

Steeply dipping rudist reef limestones, Upper Cretaceous, Bugarach, Aude, S France. Images: Roger Suthren	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 Fontfroide. Note geopetal infills in some of the rudists.	More on rudist buildups: Ocean Through Time (Smithsonian) Introduction to rudist bivalves

Tertiary

Scleractinian corals continued as a major reef former in Paleogene and Neogene 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.

Detail of colonial scleractinian corals in the Mallorca reefs

Quaternary

Scleractinian coral colonies in the Pleistocene Key Largo Limestone, Windley Key Quarry, Florida Keys	Sawn block of reef limestone from Windley Quay. Lower scale in cm.
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 (pdf fact sheet)

Modern reef images

• Coral Kingdom photo library (NOAA)
• Hawaii

This page is maintained by Roger Suthren. Last updated 16 March, 2021 15:51