4Th year in geology department

Topic- Reef

Shallow offshore ocean floor

Extends from the spring low-tide shoreline to the edge of

the continental shelfMostly sediment-covered but contains rocky exposuresIncludes:

Kelp forestsCoral reefs

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.

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

Grow to form a reef

Attacked by a number of physical and biological factors

Generates sediment

Disperse away from original reef Hydromechanical

build-up

Fills in original reef and becomes cemented

Change structure of original reef

Coral reef distribution and diversity

Figure 15-18

Coral reefs: Environmental conditionsCoral reefs need:

Warm water: 18-30°C (64-86°F)Strong sunlight (for symbiotic algae)Strong wave/current actionLack of turbiditySalt waterHard substrate for attachment

•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.

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

Coral reef zonation

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

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.

Stages of coral reef development

Figure 2-30

Comparison of the three types of reefs

-form around volcanoes

-form at the edge of land mass

-stand offshore separated from land by lagoon -outer edge is upturned

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.

How are reefs formed?

Atolls as an example

Lyell - 1832 - Principles of Geology

-Formation of Atolls-corals grew on submerged volcanoes

R.A. Daly- 1910 - Glacial Control Theory

Darwin - 1840’sTheory of Volcanic Subsidence

Growth of an Atoll

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

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.

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.

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.

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.

Reef structures

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 changeSea 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.

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)