Lecture 1

Reef Types and Formation

Bonaire. N.A.

Scope of the course

Reef

Mangrove

Prevail

ing winds

“Wild Side”

Reef - any upthrusting substrate in the ocean(generally rock or coral)

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

Carbonate Reefs

-reefs comprised principally of CaCO3

(have been defined as “aquatic biosedimentary structures”)

Mud mounds Primary FrameworkReefs

Problems in Classification of Carbonate “Reefs”

Matrix

Cavity + cementIn-place skeletons

Problems in Classification of Carbonate “Reefs”

Ecological abilities of organisms to build reefs

Changes in sedimentation, temperature & salinity

Wave energy

Sea level changes

Differences in rate and nature of construction

and destruction of a reef

Development of different reef types

Classification of Carbonate Reefs

Points of agreement

• carbonate reefs owe their origin to biological secretion of CaCO3

• rigid structures that stand above the ocean floor

• exert some control on local oceanographic processes

• historically, reef-builders are in several different taxonomic groups

Classification of Carbonate Reefs

Points of Disagreement

• centred on the ‘framework’ of a reef

-a ‘true’ reef in-place and organized interlocking meshwork of skeletons providing rigidity

Problems in Classification of Carbonate “Reefs”

10’s of kms.

Sea level 1

Sea level 2

-high rates of sediment accumulation, infilling

-sediment flushing, less infilling

Classification of Carbonate Reefs

Points of Disagreement

• centred on the ‘framework’ of a reef

Ancient Reefs

Carbonate mass

- recognizable organisms but loosely packed and in detrital matrix

Coral Reef - compacted and cemented assemblage of skeletons and skeletal sediment of sedentary organisms living in warmmarine water with strong illumination

100% Displaced Framework

100% In-place Framework

Hydromechanical Build-Up

2º Framew

ork Reef

1º Framew

ork Reef

Increasing Secondary Encrusters and Cement

Sedi

men

tary

Bui

ldupBaffling

Reefs

Mud mounds,stromatolites

Sand bars,Spits

% Sediment

Types of Coral Reefs

1. Atolls - ring or horseshoe shaped

Types of Coral Reefs1. Atolls -

Types of Coral Reefs

2. Fringing Reefs - built on margins of land

Types of Coral Reefs

2. Barrier Reefs - Separated from land

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 lagoon

Submerged Shelf Edge Reef

Summary - Major Reef Types

Comparison of the types of reefs

Type Shape Location Emergent? Lagoon

Fringing Linear Near shore yes < 500 m wide < 5 m deep

Barrier Linear Dictated by yes > 500 m wide

oceanography > 5 m deep

Atoll Circular Platform margin yes central: deep

Patch Reef ≈ Equant Lagoon can be N/A

Shelf Edge Linear At shelf edge no N/A

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

Drilling at Enewetok Atoll

Drilling at Eniwetok Atoll

Why????

Drilling at Enewetok Atoll

Drill core

Coral skeletonmatrix(CaCO3)

Igneous(volcanic)rock

Ca. 1400 m

Atoll Structure

volcanic rock

limestone & dolomite (CaCO3)

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