reef 2_Shallow offshore ocean floor 3_Reef - any up thrusting substrate in the ocean 4_Organism which form reefs 5_Coral reefs: Environmental conditions 6_Coral reef zonation 7_Reef settings : Types of Coral Reefs 8_ How are reefs formed? 9_ Reef structures 10_ Reef characteristics 11_ Economic geology of anciant reefs

1. 4
Th
year in geology
department
Topic- Reef

2. Shallow offshore ocean floor
Extends from the spring
low-tide shoreline to the edge of
the continental shelf
Mostly sediment-covered
but contains rocky exposures
:Includes
Kelp forests 
Coral reefs 
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3. Reef - any up thrusting substrate in the ocean
(generally rock or coral)
Reefs are carbonate bodies built up mainly by
framework- building benthic organisms such as
corals. They are wave resistant structures that
form in shallow waters on carbonate platforms.
The term ‘reef’ is used by mariners to indicate
shallow rocky areas at sea, but in geological
terms they are exclusively biological features.

4. Organism which form reefs: - skeletons and skeletal
sediment of inactive organisms living in warm marine water with strong illumination .Also
AS plank tonic organism ( foraminifera , roccolthiphorids , radiolaria , diatoms , calcareous
algae & sponges and finally corals Carbonate Reefs
-reefs comprised principally of CaCO3
(have been defined as “aquatic biosedimentary structures”)
Formed by some creature
Change
structure of
original reef
Grow to form
a reef
Fills in original
reef and becomes cemented
Attacked by a number of
physical and biological
factors
Generates sediment
Disperse away from
original reef
Hydromechanical
build-up

5. Coral reefs: Environmental conditions
Coral reefs need:
Warm water: 18-30°C (64-86°F)
Strong sunlight (for symbiotic algae)
Strong wave/current action
Lack of turbidity
Salt water
Hard substrate for attachment
Coral reef distribution
and diversity
Figure 15-18
•Coral reefs found in shallow, tropical
waters
•Australia’s Great Barrier Reef is the
world’s largest natural made
structure (2000km), and contains over
1500 species of fish.

6. Coral reef zonation
Reef settings
Types of Coral Reefs :Three main forms of reef have been recognized in
modern oceans.
1. Atolls - ring or horseshoe
shaped ,or when the reef
taking circular shape with
central lagoon

7. Types of Coral Reefs
.2. Fringing Reefs - built on margins of land
Fringing reefs are built out directly from the shoreline and lack an
extensive back-reef lagoonal area

8. Types of Coral Reefs
2. Barrier Reefs - is parallel to beach line and lagoons
Separated from land
Barrier reefs, of which the Great Barrier Reef of eastern
Australia is a distinctive example, are linear reef forms
that parallel the shoreline, but lie at a distance of
kilometers' to tens of kilometers offshore: they create
a back-reef lagoon area which is a large area of
shallow, low-energy sea, which is itself an important
ecosystem and depositional setting.

9. Stages of coral reef development
Figure 2-30

10. Comparison of the three types of reefs
-form at the edge of
land mass
-form around
volcanoes
-stand offshore
separated from
land by lagoon
-outer edge is
upturned

11. Patch Reef
-isolated coral community - often in
lagoonPatch reefs: In open ocean areas coral
atolls develop on localised areas of shallow
water, such as seamounts, which are the
submerged remains of volcanic islands. In
addition to these settings of reef formation,
evidence from the stratigraphic record indicates
that there are many examples of patch reefs,
localised build-ups in shallow water areas such
as epicontinental seas, carbonate platforms and
lagoons.

12. How are reefs formed?
Atolls as an example

13. Lyell - 1832 - Principles of Geology
-Formation of Atolls
-corals grew on submerged volcanoes

14. R.A. Daly- 1910 - Glacial Control Theory

15. Darwin - 1840’s
Theory of Volcanic Subsidence

16. Growth of an Atoll

17. In the process of atoll formation, the three kinds
of reefs grade into one another

18. Reef structures
Modern reefs can be divided into a number of distinct subenvironments.
The reef crest is the site of growth of the corals that build the most robust
structures, encrusting and massive forms capable of withstanding the
force of waves in very shallow water.

19. Reef structures
Going down the reef front these massive and encrusting forms of coral
are replaced by branching and more delicate plate-like forms in the lower
energy, deeper water.
Behind the reef crest is a reef flat, also comprising relatively robust forms,
but conditions become quieter close to the back-reef area and globular
coral forms are common in this region.

20. Reef structures
Break-up of the reef core material by wave and storm action leads to the
formation of a talus slope of reefal debris. This forereef setting is a region
of accumulation of carbonate breccia to form bioclastic rudstone and
grainstone facies.

21. Reef structures
Behind the reef crest the back reef is sheltered from the highest energy
conditions and is the site of deposition of debris removed from the reef
core and washed towards the lagoon.
A gradation from rudstone to grainstone deposits of broken reef material,
shells and occasionally ooids forms a fringe along the margin of the
lagoon.

22. Reef characteristics
.Found in tropical and sub-tropical zones
In the present day reef growing in shallow tropical sea , and don’t grow above the
.sea level because it killed as soon as exposure to air & photic zone
The best of reef form is less than 25 fathoms of water , salinity between 27_40 ppt
. and temperature
The shape and the growth of reef depends on sea level change
.Sea static
reef towards the sea
.Slow rise
reef towards land up ward
.Rapid rise
reef killed because of absence the light
.Slow subsidence
reef towards open sea &down ward
High porosity when it formed and during formation and compaction degree is weak so
.it keep it’s primary porosity
.It forms of un stable minerals calcite & aragonite
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23. Economic geology of anciant reefs
because of there porosity they make hydrocarbon
reservoir (oil & gas) .
there porosity & chemical instability they allowable to
replacement by metallic ore minerals (the host to
)mretalliferous deposits
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