Lecture 18: Biogenic build-ups: the reefs they are a-changin'... Flashcards
When was phanerozoic time?
Cambrian to anthropogenal
What is a reef?
A biological build up- a number of organisms growing together to build a structure.
What is the framework of a reef?
Mineralised invertebrate skeletons and non-skeletal organisms which induce carbonate precipitation (e.g., bacteria)
What are patch reefs?
Small and isolated things
What are fringing reef?
Linear, subtidal near land
What are barrier reef?
Linear, separated from land by wide/deep lagoon
What shape are atolls?
Horseshoe or ring-shaped
If laterally extensive reef complexes have relief, they develop distinct zones…?
Fore reef, talus slope
Back reef
Lagoon
What is the basic framework of a reef?
Primary community: mutual interconnection
Secondary community: encrusts
Between these are cavities or ‘crypts’- cryptic faunas
This can fill with sediment- precipitation of carbonate cement
Environmental factors that shape reefs…
Water motion
Depth (light and water motion)
Sedimentation rate/ proximity to land
Relative sea level change - eustatic (global) and isostatic (local)
Reefs are rate after each extinction and there has been several crises and recoveries. What happens during recovery?
Reef communities have changed through time because during recovery, the community reassembles.
New reef complexes dominated by new organisms each time.
What were reefs like in the precambrian?
Simple
Stromatolites
Rare after precambrian (eaten by animals…)
Morphology related to depth and energy level.
Extinction- metazoan arrived
What were reefs like in the cambrian?
Arrival of new…
Archaeocyathids, vase like sponges which is skeletised.
They are restricted globally- get them in southern hemisphere.
Calcifying cyanoabacteric form mounds with crypts.
Some sponges and early crinoids
These increased the diversity.
No corals
What were reefs like in the mid-cambrian?
EXTINCTION
No metazoan reefs Late Cambrian early ordovician
What were reefs like in the mid-ordovician-devonian?
The first corals…‘anemones with skeletons’
Two sorts of corals…
- Tabulate
- Rugose (horn)
Bryozoans- ‘moss animals’
Stromatoporoids- calcareous sponges
What was the Devonian reef maximum?
Reefs found across wider geographic area and higher latitudes than any other time in Earth history
What happened in the late Devonian?
Late Devonian extinction
Carboniferous reorganisation leaves reefs with not so much corals but instead bacterial structures.
What were reefs like in the Permian (Capitan reef)?
Enormous Texan Reef complex, Delaware Basin
Unusual biota
Shallow water: platy calcareous sponges
Deeper water: frondose bryozoans
What was the Permo-Triassic extinction?
No metazoan reefs in Early Triassic.
Stromatolites still around.
What happened in the late Triassic reorganisation?
A new coral group evolves.
The new scleractinians/hexacorals
Aragonite- very soluble (doesn’t last)
‘Modern corals’ with symbiotic algae
What is the modern tropical reef?
Still dominated by scleractinian framebuilders
Soft coral present
Little different from jurassic
Fossil calendars
Corals show daily growth bands and a more pronounced yearly band.
