Lecture 1 Reef Types and Formation. Bonaire. N.A.
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Transcript of Lecture 1 Reef Types and Formation. Bonaire. N.A.
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