Lecture Thirty One - Shelf environments and carbonate rocks Flashcards
What are carbonates?
Carbonate facies are:
Any rock >50% carbonate minerals
- Calcite (hexagonal): CaCO3
- Aragonite (orthorhombic): CaCO3
–> Calcite & aragonite occur as marine invertebrate skeletons
- Dolomite (hexagonal): CaMg(CO3)
–> Dolomite (almost) always diagenetic (replacement of calcite)
Divided into limestone and “dolostone” on basis of mineralogy.
Reefs <5% of all limestone
Carbonates are excellent palaeoenvironmental & palaeoclimatic indicators
Carbonates are excellent sources and reservoirs for hydrocarbon
How are carbonates formed organically and inorganically?
Carbon is extracted from seawater to form carbonates by both organic or purely inorganic processes
Organic: building skeletons, i.e., shells or tests (most important mechanism)
–> Bioclastic debris
Inorganic: direct precipitation from
seawater
E.g. Ooids, whiting
What is whiting?
Occurs in lakes when the temperature increases thus causing CaCO3 to precipitate out of solution and make the lake turn white.
Occurred e.g. in Lake Michigan in the USA.
What are the factors that control the rate of carbonate accumulation?
Factors that control the rate of carbonate accumulation:
1) Wave agitation: promotes oxygenation of waters (and carbonate production).
2) Sea level: slow & steady sea level rise promotes carbonate production
(sea level drop
What are pelagic ooze sediments?
Calcareous ooze = ‘chalk’ forms at depths less than the CCD.
Siliceous ooze = ‘chert’ forms at depths deeper than the CCD.
Pelagic oozes = Composed of greater than 30% debris of planktonic (free floating) organisms.
–> Specific composition dependant on the distance from landmass, oceanic productivity and the depth.
Why is there a reef-less patch off Africa where conditions seem to be favourable for reef formation?
Reef dead patch off Africa because there are currents moving warm water away from
this area and cold currents are being moved up along the western coast. Making water too cold for reef formation.
Where is a good place for reef formation?
North East of Australia (Great Barrier Reef).
What are the sequence of shallow water carbonate structures?
Evolving sequences dependant on climate, water depth and energy conditions.
1) Shoals
Shallow - moderate depths (shallower than 15m; sub tidal zone).
Low energy, current generated sedimentation.
Low profile rise of bioclastic sediment on shelf.
2)Mounds
Mound of bioclastic sediment and in situ sediment-trapping (‘sessile’/immobile) organisms.
E.g. Scattered crinoids, bryozoa, corals.
- Generated by currents & biogenic growth.
- Periodically disrupted by storm events.
- Moderate to shallow water, low energy.
3)Reefs
Dense community of robust, skeletal framework organisms.
- Massive and branching corals, bryozoa, stromatoporoids, red encrusting algae (depending on time period).
High energy, shallow water, warm temperature settings - tropical latitudes.
Range of reef types - specific reeds dependent on shelf topography and reef structure.
What is the relationship between feed ‘builders’ and ‘planners.’
There is a mutualistic relationship between reef ‘builders’ (e.g. corals) and reef ‘planners’ (photosynthetic ‘zooxanthellae.’)
Reef builders provide CO2 and protection.
Zooxanthellae (e.g. algar) provide oxygen and nutrients.
Active carbonate structures (e.g. reefs) are a complex interplay of feedbacks between organisms and the environment they inhabit.
–> Actively transforming the environment to suit them.
By progressive stacking of carbonate sediments higher into the lucrative.
What is the structure of a reef controlled by?
Size, morphology and relation to landmass.
1) Barrier reef:
Long (100’s to 1000’s of km).
Form along continental shel edge.
2) Fringing reefs:
Form along edge of continent or island, next to shoreline.
- Minimal back-reef lagoon (reef is more or less hard up against landmass).
3) Atoll reefs:
Isolated reefs, commonly form around submerged volcanic islands (seamounts/guyots).
Reef borders a lagoon.
4) Patch/pinnacle/table reefs
Small isolated reefs, commonly form on shelf.
Look in notes for diagrams of reefs - should know how to label one e.g. fore reef, back reed, and reef front etc.
What are the different types of carbonate platforms?
1) Rimmed shelf (shelf edge model).
Shelf edge shallowing abruptly focuses wave energy and nutrients from deep water upwelling.
Rapud lateral change in energy conditions.
Oxygenates nutrient rich water.
After reef reaches upper limit (SL) - spreads laterally forming a wider shelf.
2) Sloping shelf ramp.
Gently sloping shelf - gradual lateral facies changes.
No abrupt energy change - no focus of upwelling/oxygenation.
generally lower energy settings.
Mounds or patch reefs occur.
No pronounced barrier reefs.
3) Epeiric platform/epicontinental seas (drowned continent - scale difference).
Vast platform in photic zone.
the setting of most carbonate rocks formed in geological history (by volume).
More common during global greenhouse events (‘SL high stands’).
Look in notes at this diagram to get idea of how to label these types of reefs.
Describe temperate shelf limestones.
Cooler water (less than 20 degrees C) limit abundance and diversity of reef builders.
Dominant cooler water species.
- Bryozoans, molluscs, brachiopods, foraminifera, echinoids, solitary corals.
- Sparse colonial corals.
- Small colonies distributed over the shelf.
- Episodically hit by major storms.
–> Erode and breakup organisms into bioclastic debris.
Ancient sequences characterised by alternating beds of:
Bioclastic debris and ‘marl’ = mud sized sediment (~50% carbonate: 5% silicate).
Describe tropical shelf limestones.
Warm tropical climate carbonates:
High T (>20oC water T).
Preferable to all organisms.
Corals and other rapidly growing, robust primary producers dominate (23-27oC).
–> Corals may occur in waters below 10oC but they are not reef forming.
Reefs prominent + bioclastic shelf sands.
Describe open ocean deep water pelagic carbonates.
Open ocean deep-water pelagic carbonates:
Pelagic oozes.
Remains of calcareous planktonic
(‘free-floating’) organisms.
Skeletons settle to seafloor.
Preserved in low-middle latitudes
–> Siliceous oozes (chert) more common at higher latitudes.
At water depths < carbonate compensation depth.
–> Precipitation/sedimentation of
CaCO3 (vs. siliceous oozes).
