Lecture 1 Reef Types and Formation

Bonaire. N.A.

Scope of the course Reef Mangrove Prevailing winds “Wild Side”

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

Carbonate Reefs -reefs comprised principally of CaCO 3 (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 CaCO 3 (have been defined as “aquatic biosedimentary structures”) Mud mounds Primary Framework Reefs

Problems in Classification of Carbonate “Reefs” Matrix Cavity + cement In-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 CaCO 3 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 warm marine water with strong illumination

100% Displaced Framework 100% In-place Framework Hydromechanical Build-Up 2º Framework Reef 1º Framework Reef Increasing Secondary Encrusters and Cement Sedimentary Buildup Baffling Reefs Mud mounds, stromatolites Sand bars,Spits % Sediment

Types of Coral Reefs 1.Atolls - ring or horseshoe shaped

Types of Coral Reefs 1.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 Principles of Geology -Formation of Atolls -corals grew on submerged volcanoes

R.A. Daly Glacial Control Theory

Darwin ’s Theory of Volcanic Subsidence

Growth of an Atoll

Drilling at Enewetok Atoll

Drilling at Eniwetok Atoll Why????

Drilling at Enewetok Atoll Drill core Coral skeleton matrix (CaCO 3 ) Igneous (volcanic) rock Ca m

Atoll Structure volcanic rock limestone & dolomite (CaCO 3 )

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