Carbonate margins and reefs

Question 1

Define carbonate build-ups

Answer 1

Laterally-restricted structures that have usually undergone organically-mediated growth

Question 2

What are the two subdivision of carbonate build ups?

Answer 2

Organic (skeletal) reefs
Reef (mud) mounds

Question 3

Describe organic (skeletal) reefs

Answer 3

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

Question 4

Describe reef (mud) mounds

Answer 4

Inorganically and/or biogenically constructed
Lack rigid skeletal framework and unable to withstand high energy wind/wave action

Question 5

Describe the structure of mud mounds

Answer 5

Mud (micrite)-dominated
Stability provided by matrix
Limited cementation
Low topographic relief

Question 6

When were mud mounds common and why?

Answer 6

Palaeozoic
Skeletal frame builders were absent or in deeper water

Question 7

Give examples of organic components in mud mounds

Answer 7

Bivalves, corals, sponges, bryozo, microbrobes

Question 8

Where do reefs preferentially develop?

Answer 8

On topographic highs
e.g. older reefs, karst, volcanic features

Question 9

Describe the constructive processes of reefs

Answer 9

Biological processes though direct growth, baffling, or binding

Question 10

Describe destructive processes of reefs

Answer 10

Wave damage and biological destruction

Question 11

Describe cementation of reefs

Answer 11

Early cementation from sea water

Question 12

Describe sedimentation of reefs

Answer 12

Accumulation of biogenic matter and reef-derived detritus

Question 13

What is the most contributing type of organism in frame built carbonates?

Answer 13

Frame-builders
Corals

Question 14

What is the most contributing type of organism in mud mounds?

Answer 14

Precipitators
Microbial mats

Question 15

What are the four stages of the Oxfordian reef ecological succession?

Answer 15

Pioneer, colonisation, diversification, domination

Question 16

What conditions are required for delicate, branching growth forms?

Answer 16

Low wave energy and high sedimentation

Question 17

What conditions are required for globular and columnar growth forms?

Answer 17

Moderate wave energy and high sedimentation

Question 18

What conditions are required for encrusting growth forms?

Answer 18

Intense wave energy and low sedimentation

Question 19

What facies are seen on the back reef?

Answer 19

Bafflestone, floatstone

Question 20

What facies are seen on the reef flat?

Answer 20

Rudstone, grainstone

Question 21

What facies are seen on the reef crest?

Answer 21

Bindstone

Question 22

What facies are seen on the reef front?

Answer 22

Framstone, bindstone, bafflestone

Question 23

What facies are seen on the fore reef?

Answer 23

Grainstone, rudstone

Question 24

What types of growth forms are seen on the reef front? In order as you go down the front

Answer 24

Encrusting, massive, branching, plate-like

Question 25

Describe the reef front

Answer 25

Highest point on the reef to the seaward depth where little/no frame-building (up to 100m)

Question 26

Describe the reef crest

Answer 26

The highest, most exposed part of the reef
Very high energy (bioerosion and skeletal breakage is high)

Question 27

Describe the organisms at the reef crest

Answer 27

Dominated by encrusting organisms
e.g. red algae, usually coating dead coral/coral debris
May be encrusted by forams, gastropods

Question 28

Why is there low preservation potential at the reef front?

Answer 28

Bioerosion and early diagenesis

Question 29

Describe the structures on the reef front

Answer 29

In the higher energy zones, spur and groove structures form oblique to the shoreline

Question 30

Describe sedimentation at the fore reef slope

Answer 30

Positioned seaward of the reef front, transition to basin
Sedimentation dominated by gravity flow and deposition of pelagic sediments

Question 31

Describe the reef flat

Answer 31

Sand apron
Reworked reef debris and carbonate sand
Colonisation by sea grass and algal mats
Some coral growth

Question 32

What has caused the change in the composition of skeletal components of carbonates through time?

Answer 32

Evolution, extinction events, ocean chemistry, continental configuration