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Carbonate Reefs and Build-ups 3

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.

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

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.

 

Lyngbya

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:

St Chinian - Lias stromatolite - 54 kb

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:

Boutenac - algal laminites

(Lower Jurassic, S France)

For example, they are well laminated on a mm or cm scale.

Polished surface of simple laminite (left). Similar structures are seen in freshwater tufas, many of which are now interpreted as stromatolites (i.e. the products of microbial mats).

 

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.

Oncolites - St Chinian

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.

 

St Chinian oncolites These examples are from non-marine Upper Cretaceous strata in southern France.

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

Cambrian


Ordovician

Tabulate & rugose corals became common. Stromatoporoids appeared.


Silurian

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

Favosites - 21 kb

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


Devonian

Meuse valley reef - 37 kb

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

Stromatactis

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:

 

 


Carboniferous

Lithostrotion

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

 


Permian

Sponges & calcareous algae dominated.


Triassic


Jurassic


Cretaceous

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.

Fontfroide rudists

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)

Fontfroide rudists 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.

Cap Blanc, 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

Scleractinian corals, together with red calcareous algae, dominate the frameworks of modern reefs.
Windley Key reef

 

Cross-section through a brain coral in the Pleistocene Key Largo Limestone, Windley Key Quarry, Florida Keys

modern reef Modern reef at Key Largo Dry Rocks, Florida Keys. Note the large, massive scleractinian coral colonies

Modern reefs

Modern reef images


Lecture 1: Introduction

Lecture 2: Modern Reefs and Buildups

Lecture 3: Reefs and Buildups through Time

 

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This page is maintained by Roger Suthren. Last updated 30 April, 2015 22:09