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EES 450: Sedimentary Geology

CARBONATES

- Rocks with > 50% carbonate materials; most abundant biochemical/chemical rocks.

- Make up 20-25% of the sedimentary record from the Precambrian to the Recent.

- Limestone and dolomite include > 90% of all carbonates.

- The ratio of dolomite/limestone increases with age. Why?

- Most dolomite is secondary or diagenetic.

- CaCO3 is polymorphic (calcite vs. aragonite).

- Most carbonates are autochthonous and contain much information about their depositional environments.

- Carbonates have significant industrial and agricultural uses.

- 1/3 of all hydrocarbon reservoirs are in carbonates.

LimestoneChertDiatomite

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BrecciaConglomerateDiamictiteSandstoneSiltstoneShale

New ClayMinerals Shale

Clastic or

Detrital

Biochemical

EvaporitesChertLimestoneIronstone

Chemical

Solution

Plant Extraction

PeatCoalBitumenResins

Organic

MechanicalWeathering

Chemical Weathering

Source Rocks

Crustal, upper mantle melts

Explosive Eruption

Tuff, BentoniteAgglomerate

Volcaniclastic

EES 450: Sedimentary Geology

CARBONATES Rudites

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EES 450: Sedimentary Geology

CARBONATES

• Why do we have carbonates?

- Earth’s atmosphere has abundant CO2, and CO2 with H2O is a powerful weathering agent.

- Feldspar (or ferromagnesian minerals) + CO2 + H2O clay minerals + cations (Ca2+, Mg2+, etc.) + HCO3

1- + silica + (Fe oxides).

- Ca and Mg are provided by the weathering of some feldspars and ferromagnesian minerals.

- CO2 is provided to Earth’s atmosphere by volcanism, the decay of organic matter, the respiration of organisms and increasingly, from human activities.

- Summary: We have carbonates because of the abundance of Ca2+, Mg2+, and CO2.

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EES 450: Sedimentary Geology

CARBONATES

• Why do we have carbonates?

- The production, dissolution and preservation of carbonate sediment are intimately associated with the global carbon cycle.

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EES 450: Sedimentary Geology

CARBONATES

• Depositional conditions.

(1) Shallow, subtropical seas.(2) Warm, evaporative conditions.(3) Minimal influx of clastic sediments.

H2O + CO2 + CaCO3 Ca+2 + 2HCO3-1

Modern day carbonate deposition.

Modern day coral reef distribution. Why do we find reefs where we do? Heavily threatened,

why?

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EES 450: Sedimentary Geology

CARBONATES

• Depositional conditions.

H2O + CO2 + CaCO3 Ca+2 + 2HCO3-1

- The distribution of carbonate sediments is controlled by the carbonate compensation depth (CCD) and the rate of surface production. Does the depth of the CCD change over time? What controls this?

- Hypsometry = measurement of landelevation relative to sea level.

- Lysocline = depth in the ocean belowwhich the rate of calcite dissolutionIncreases dramatically.

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EES 450: Sedimentary Geology

CARBONATES

• Depositional conditions.

H2O + CO2 + CaCO3 Ca+2 + 2HCO3-1

- The distribution of carbonate sediments is controlled by the carbonate compensation depth (CCD) and the rate of surface production.

NOAA Sea WIFS data, showing the surface ocean distribution of chlorophyl a; what does this tell us about the distribution of surface ocean productivity? Does this change over time? How and why?

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EES 450: Sedimentary Geology

CARBONATES

• Depositional conditions.

- Does global climate effect carbonate deposition?

Global climate belts are controlled by latitudinal variations in wind.

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EES 450: Sedimentary Geology

CARBONATES

• Depositional conditions.

- Does global climate effect carbonate deposition?

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EES 450: Sedimentary Geology

CARBONATES

• Depositional conditions – Example modern day environments.

Bahama platform.

Joulters Cays.

Persian Gulf.

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EES 450: Sedimentary Geology

CARBONATES

• Depositional conditions – Example modern day environments.

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EES 450: Sedimentary Geology

CARBONATES

• Most carbonates are biochemical in origin.

BIOLOGICAL BIOLOGICALLY INDUCED PHYSICO-CHEMICAL

Fossils Carbonate muds Cements, rocks

Hydrophytes (carbonate

encrustation).

Pteropod Limacina Helicina

(aragonite).

Foraminifera(carbonate).

Bryozoan fossils (carbonate).

Stromatolites (bacterial mats).

Fossilized coral (carbonate).

Ooids.

Travertine.

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EES 450: Sedimentary Geology

CARBONATES

• General carbonate facts:

- Carbonates are products of specific depositional environments.

- Carbonate deposition can be fast, but intermittent and “sensitive”. What does this mean?

- Carbonate deposition is generally autochthonous. Carbonate particles generally produced in situ with little net transport occurring.

- Carbonate formation is mostly biochemical.

- Basin configuration and energy setting are the predominant controls on carbonate deposition.

- Carbonates are highly susceptible to diagenetic alteration.