Carbonate margins and reefs Flashcards
Define carbonate build-ups
Laterally-restricted structures that have usually undergone organically-mediated growth
What are the two subdivision of carbonate build ups?
Organic (skeletal) reefs
Reef (mud) mounds
Describe organic (skeletal) reefs
Built by organisms with a rigid calcareous frame
Matrix or skeleton supported
Deposited in warm/cold water
Able to withstand high energy wind/wave action
Describe reef (mud) mounds
Inorganically and/or biogenically constructed
Lack rigid skeletal framework and unable to withstand high energy wind/wave action
Describe the structure of mud mounds
Mud (micrite)-dominated
Stability provided by matrix
Limited cementation
Low topographic relief
When were mud mounds common and why?
Palaeozoic
Skeletal frame builders were absent or in deeper water
Give examples of organic components in mud mounds
Bivalves, corals, sponges, bryozo, microbrobes
Where do reefs preferentially develop?
On topographic highs
e.g. older reefs, karst, volcanic features
Describe the constructive processes of reefs
Biological processes though direct growth, baffling, or binding
Describe destructive processes of reefs
Wave damage and biological destruction
Describe cementation of reefs
Early cementation from sea water
Describe sedimentation of reefs
Accumulation of biogenic matter and reef-derived detritus
What is the most contributing type of organism in frame built carbonates?
Frame-builders
Corals
What is the most contributing type of organism in mud mounds?
Precipitators
Microbial mats
What are the four stages of the Oxfordian reef ecological succession?
Pioneer, colonisation, diversification, domination
What conditions are required for delicate, branching growth forms?
Low wave energy and high sedimentation
What conditions are required for globular and columnar growth forms?
Moderate wave energy and high sedimentation
What conditions are required for encrusting growth forms?
Intense wave energy and low sedimentation
What facies are seen on the back reef?
Bafflestone, floatstone
What facies are seen on the reef flat?
Rudstone, grainstone
What facies are seen on the reef crest?
Bindstone
What facies are seen on the reef front?
Framstone, bindstone, bafflestone
What facies are seen on the fore reef?
Grainstone, rudstone
What types of growth forms are seen on the reef front? In order as you go down the front
Encrusting, massive, branching, plate-like
Describe the reef front
Highest point on the reef to the seaward depth where little/no frame-building (up to 100m)
Describe the reef crest
The highest, most exposed part of the reef
Very high energy (bioerosion and skeletal breakage is high)
Describe the organisms at the reef crest
Dominated by encrusting organisms
e.g. red algae, usually coating dead coral/coral debris
May be encrusted by forams, gastropods
Why is there low preservation potential at the reef front?
Bioerosion and early diagenesis
Describe the structures on the reef front
In the higher energy zones, spur and groove structures form oblique to the shoreline
Describe sedimentation at the fore reef slope
Positioned seaward of the reef front, transition to basin
Sedimentation dominated by gravity flow and deposition of pelagic sediments
Describe the reef flat
Sand apron
Reworked reef debris and carbonate sand
Colonisation by sea grass and algal mats
Some coral growth
What has caused the change in the composition of skeletal components of carbonates through time?
Evolution, extinction events, ocean chemistry, continental configuration
