lecture: carbonate rocks / carbonate environmentsjzachos/migrated/eart120/lectures_08/lect... ·...
TRANSCRIPT
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Lecture: Carbonate Rocks /Lecture: Carbonate Rocks /Carbonate EnvironmentsCarbonate Environments
• Ocean Carbonate Chemistry• Pelagic Carbonates
• Shallow Marine Carbonates
• Marble, limestone, chalk, ooze• Dominant component of ancient shallow
water seas & sedimentary rocks (warm)• Autochthonous - as opposed to allocthonous• Fossiliferous - records evolution of life• Economic importance
– Lime - cement– Reefs - porous: reservoirs for oil or for ground
water.
Carbonate Sediments
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Cretaceous/Cenozoic Reef Complex
Seawater Carbonate Chemistry and BufferingCO2 + H2O = H2CO3
• More CO2 is absorbed by sea-water than other gases• Charge Balance: if pH decreases (more acidic) CO3 ion
reacts with H ion...
CO2 + H2O = H2CO3
H2CO3 = H+ + HCO3
-
HCO3- = H + + CO3
2-
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• Dissolved Inorganic Carbon (DIC) = HCO3- + CO3
2-
– increases w/depth (1950 to 2200 µmol/kg)– 50 to 60x atmosphere
Biological/Physical Processes
1. Density Stratification• Decreasing T
2. Photosynthesis -Surface Ocean (photiczone):CO2 + H2O = CH2O + O2
3. Respiration -DeepOcean:CH2O + O2 = CO2 + H2O
Pelagic Carbonate1. Phytoplankton - unicellular
algae-photosynthetic (<.070mm)– base of the marine food chain
(grasses of the sea)• Calcareous Algae
– COCCOLITHS - calcite plates– warm, low nutrient water
• Coccolith ooze• Chalk
– fine grain (micritic)
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Pelagic Carbonate2. Planktonic Foraminifera - (0.70 to 1 mm)
– Protozoa - single cell organisms– primarily grazers, harbor photosymbionts– Calcite shells (tests)
Deep Sea [CO3] & calcite saturation (Ω) or compensation depth (CSD/CCD)
40 80 120 160 200 240 [CO3] (µmol/kg)
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2
3
4
0
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Depth (km)
calcite saturation
aragonite saturation
CaCO3
Clay
GEOSECS, S. ATLANTIC
Primary control ondistribution of carbonatesediments?
• CO3 & Calcite Saturation(Ω=1)
[CO3] = 50 µmol/kg at 0 km [CO3] = 95 µmol/kg at 5 km
• Calcite CompensationDepth
[CO3]WE> [CO3]SAT
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GEOSECS Global [CO3=] - T data
[CO3=] and T are correlated on a global scale…
Pelagic Biogenic & Clay SedimentOpal facies - upwelling
regions• East eq. pacific• Circum Antarctic
Clay facies• Atlantic >4 km• Pacific >3 km
Vertical Successions?• Carbonate to clay
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% CaCO3 at the Modern Lysocline
Broecker 1999
Lateral Variations in[CO3] & CaCO3
• Deep Sea Circulation• Biological pump -
respiration of CO2– deep water massesAccumulate CO2
–Lowers pH and CO3
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Zachos et al., 2005
Siliceous Sediments/RocksComposed predominantly of SiO2
minerals quartz, chalcedony,and opal + minor impurities
• Biogenic Silica - amorphousSilica/Opal A (SiO2*H2O)– DIATOMS - Algae– RADIOLARIA - Zooplankton– opal to chert
• Chert - microcrystallinequartz, w/minorcalcedony/opal– Grain sizes/shapes variable (1-50
µm)
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Dissolved Silica DistributionAmorphous SiO2 - highly soluble• Seawater (H4SiO4)
– <200 µmol/kg– Highly undersaturated!– Organic coatings preserve shell
opal– Accumulation occurs only where
fluxes are high– Diatom/radiolarian oozes
Origin of Biogenic Sediment
1. Silica Source:Upwelling zones-high
productivity (diatoms)
Chlorophyll contents inthe Pacific
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Franciscan Chert• Cretaceous Radiolarian• Upwelling• Deep water• Low latitude
Corona Heights Park, San FranciscoMarin Headlands
Shatsky Rise, Pacific
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Deep Sea [CO3] & calcite saturation or compensation depth (CSD/CCD)
40 80 120 160 200 240 [CO3] (µmol/kg)
1
2
3
4
0
5
Depth (km)
calcite saturation
aragonite saturation
CaCO3
Clay
GEOSECS, S. ATLANTIC
Calcite Saturation• [CO3] = 50 µmol/kg at 0 km• [CO3] = 95 µmol/kg at 5 km
Calcite Compensation Depth[CO3]WE> [CO3]SAT
Shallow Water Carbonate Sediments
• Controlling Factors (processes)– Organisms– Light availability (Turbidity of the water, water
depth).– Nutrient availability– Water Temperature
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CO3=
+ Ca2+ CaCO3 (Carbonate)
Zooxanthellae Symbionts
Photosynthetic and nutrient recycling dinoflagellates
CO2 utilization aids in formation of coral skeleton
Carbonate grains - two categories:1. Orthochems (authigenesis, in situ)
a) Micrite: fine grained carbonate mudb) Sparite: Interlocking crystals of calcium
carbonate2. Allochems (various degree of transport)
a) Nonskeletal grains: coated grains, peloids,grain aggregates, carbonate clasts.
b) Skeletal grains: fragments of organismscorals, mollusks, echinoderms, sponges, bryozoans,
foraminifers, etc…
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Orthochems
Micrite (fine carbonate mud)
Sparites1. Fiberous or2. equigranular
Allochems: nonskeletal
Coated grains (ooids, pisoids, oncoliths)
Peloids (bacterial activity
results in micritized carbonate grains or pellets)
+ grain aggregates and carbonate clasts
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Allochems: skeletal, heterotrophs
Mollusks Bryozoans(suspension feeders)
Most benthic foraminifersEchinoderms (ursins, crinoids, etc…)
Allochems: skeletal, autotrophs
Corals
Halimeda
Rhodolith
Coccolithoforides
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Limestone classification(Folk, 1959, 1962)
• Prefixes: framework grains– bio- fossils– pel- peloids– oo- ooids– intra- intraclasts
• Stems: interstital calcite– micrite– sparite
Biomicrite Biosparite
Pelsparite?
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Textural maturity of limestone
Based on1. % of allochems present2. degree of sorting and extent of rounding
after Folk (1959, 1962)
Limestone classification (Dunham 1962)Based on primary matrix (thin sections)
– % Grains/matrix (direct relation to energy level and origin of the carbonates)
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Shallow MarineCarbonate Environments
• Carbonate Platforms
• Reefs and Buildups
• Sub-tidal Shelf Carbonates
• Peritidal
Carbonate Platforms1. Rimmed shelves - reef barrier• outer edge-pronounced break in slope (e.g.,
Australia, S. Florida Bay) - carb. sands/muds• Protected areas - lagoons, tidal flats
2. Unrimmed platforms (Ramps)• gradual slope (<2°) (e.g., W. Florida) carb.
Sands/muds
3. Isolated platforms - reef/buildup• Offshore - islands (Bahamas)
4. Epeiric platforms - ancient• Broad shallow shelves/cont. seaways
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Shelf Carbonates - Back-reef grainstones
Reefs & buildups
Cross section of a platform margin reef facies
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Types of Reefs• Shelf Reefs
– Adjacent to continents– Fringing, Barrier and Patch Reefs
• Oceanic Seamount Reefs -- Atolls– Develop on Volcanic Pinnacles– Fringing, Barrier and Patch Reefs
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Algae Structures formed in Limestones of the Paleozoic
Algae - oldest, most common
latePaleozoicClam-likeBrachiopods
MesozoicRudists(clams)
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Cretaceous/Cenozoic Reef Complex
United Arab Emirates
rudist
Reef GrowthPrimary Controls:• Sea level change
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Diagnostic features of reefs and buildups
• Small, local mound-like or bank accumulation– rapid lateral changes in facies and thicknesses.
• No typical stratigraphic sequence.• Framework builders are dominant
– entire deposit grows / bound together.• Formed entirely of fossils that determine the
growth and shape of the build-up– very restricted ecological niches
Shallowing upward sequencelow-energy carbonate shelf
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Diagnostic features of Sub-tidal ShelfCarbonates
• Requirements:– warm waters– normal salinity– light. Restricted to the continental shelves and epeiric seas in low
latitude, with no significant siliciclastic input.• thousands of square kilometers and hundred meters in
thickness.• Varied texture - pelletal muds rich in biogenic debris,
ooids, skeletal sands and bioturbated muds.• Bedding of variable thicknesses, wedge- and lens-
shapped, flaser bedding & nodular bedding.• Abundance of fossils of normal marine fauna tolerant to
limited range of salinities.– High biodiversity
Peritidal carbonate environments
Peritidal marsh
Tidal flat
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Peritidal carbonate environmentsStromatoliths inperitidal zone(Hamling Pool,Western Australia)
Sabkha environment (Persian Gulf)
Diagnostic features Peritidal Carbonates• Requirements:
1. warm waters2. normal salinity3. light– continental shelves and epeiric seas in low latitude.
• Thin but extensive beds representing shoreline facies.• Characteristic features
– stromatoliths, algal matts, mudcracks, tidal channel beccia with shellsand rip-up clasts, birdseye structure, evaporites.
• Living fossils are mainly stromatoliths & rare burrowingmollusks– shell debris washed onshore during storms.
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James, 1997: Heterozoan -> cool water, Photozoan -> warm water
Use of carbonates as paleo-proxies
Halfar et al., 2004
Limestone precipitation
�
2CO (gas) + H2O(liquide)!H2CO3
H2CO3 !H+
+ HCO3
"
HCO3
"!H
++ CO3
2"
Ca2+
+ CO3
2"!CaCO3(limestone)
k=10-1.43
k=10-6.4
k=10-10.33
k=10-8.33
k=10-8.48
(aragonite)
(calcite)
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Zachos et al., in press
Broecker et al., 1999
Present Day %CF (>63µm) / CarbonateIon Content Relationship
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Mesozoic shelf carbonates
Mesozoic shelf carbonatesCretaceous platform, Vercors, France
Western Interior -seaway