Lecture 18: Biogenic build-ups: the reefs they are a-changin'...

Question 1

When was phanerozoic time?

Answer 1

Cambrian to anthropogenal

Question 2

What is a reef?

Answer 2

A biological build up- a number of organisms growing together to build a structure.

Question 3

What is the framework of a reef?

Answer 3

Mineralised invertebrate skeletons and non-skeletal organisms which induce carbonate precipitation (e.g., bacteria)

Question 4

What are patch reefs?

Answer 4

Small and isolated things

Question 5

What are fringing reef?

Answer 5

Linear, subtidal near land

Question 6

What are barrier reef?

Answer 6

Linear, separated from land by wide/deep lagoon

Question 7

What shape are atolls?

Answer 7

Horseshoe or ring-shaped

Question 8

If laterally extensive reef complexes have relief, they develop distinct zones…?

Answer 8

Fore reef, talus slope
Back reef
Lagoon

Question 9

What is the basic framework of a reef?

Answer 9

Primary community: mutual interconnection
Secondary community: encrusts
Between these are cavities or ‘crypts’- cryptic faunas
This can fill with sediment- precipitation of carbonate cement

Question 10

Environmental factors that shape reefs…

Answer 10

Water motion
Depth (light and water motion)
Sedimentation rate/ proximity to land
Relative sea level change - eustatic (global) and isostatic (local)

Question 11

Reefs are rate after each extinction and there has been several crises and recoveries. What happens during recovery?

Answer 11

Reef communities have changed through time because during recovery, the community reassembles.
New reef complexes dominated by new organisms each time.

Question 12

What were reefs like in the precambrian?

Answer 12

Simple
Stromatolites
Rare after precambrian (eaten by animals…)
Morphology related to depth and energy level.
Extinction- metazoan arrived

Question 13

What were reefs like in the cambrian?

Arrival of new…

Answer 13

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

Question 14

What were reefs like in the mid-cambrian?

Answer 14

EXTINCTION

No metazoan reefs Late Cambrian early ordovician

Question 15

What were reefs like in the mid-ordovician-devonian?

Answer 15

The first corals…‘anemones with skeletons’

Two sorts of corals…

• Tabulate
• Rugose (horn)

Bryozoans- ‘moss animals’

Stromatoporoids- calcareous sponges

Question 16

What was the Devonian reef maximum?

Answer 16

Reefs found across wider geographic area and higher latitudes than any other time in Earth history

Question 17

What happened in the late Devonian?

Answer 17

Late Devonian extinction

Carboniferous reorganisation leaves reefs with not so much corals but instead bacterial structures.

Question 18

What were reefs like in the Permian (Capitan reef)?

Answer 18

Enormous Texan Reef complex, Delaware Basin
Unusual biota
Shallow water: platy calcareous sponges
Deeper water: frondose bryozoans

Question 19

What was the Permo-Triassic extinction?

Answer 19

No metazoan reefs in Early Triassic.

Stromatolites still around.

Question 20

What happened in the late Triassic reorganisation?

Answer 20

A new coral group evolves.
The new scleractinians/hexacorals
Aragonite- very soluble (doesn’t last)
‘Modern corals’ with symbiotic algae

Question 21

What is the modern tropical reef?

Answer 21

Still dominated by scleractinian framebuilders
Soft coral present
Little different from jurassic

Question 22

Fossil calendars

Answer 22

Corals show daily growth bands and a more pronounced yearly band.