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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-

3

• 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)

1

2

3

4

0

5

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

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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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Shatsky Rise, Pacific

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Shatsky Rise, Pacific

Paleocene-Eocene Boundary

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Zachos et al., 2005

Zachos et al., 2005

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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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Coral reef distribution

• Salinity• Turbidity

Controls on distribution

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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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Rimmed Platform

RimmedPlatform

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Unrimmed Platform

Isolated Platform

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Subtidal shelf carbonates

Isolated Platform

Ooid sands -inorganic

Halimeda sands -organic

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Permian Basin Reef

Guadalupe Mts, West Texas

El Capitan (reef)

Basin sandstones

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Permian Basin Reef

Guadalupe Mts, West Texas

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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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The Great Barrier Reef, Queensland, Australia

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Barrier Reef and Lagoon with development of small Patch Reefs

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Reef builders through time

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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

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High Energy Carbonate Shelf

Shoaling upward sequence