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EES 450: Sedimentary Geology
CARBONATE MATRIX
• Matrix in a carbonate sediment consists of very fine grained carbonate material filling the interstices between grains. It is largely synsedimentary and not chemically precipitated. It is commonly referred to as micrite (microcrystalline calcite), carbonate mud or lime mud.
GrainMicrite
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EES 450: Sedimentary Geology
CARBONATE MATRIX
• A major source of micrite is the disintegration of calcareous green algae(Chlorophyta).
Calcareous green alga Halimeda.
Carbonate mud (micrite).
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EES 450: Sedimentary Geology
CARBONATE MATRIX
• Formation of carbonate mud (micrite).
- Green algae production = 50 g y-1 m-2.
- Brown algae production = 240 g y-1 m-2.
Live green algae.
Bleached algal fragments.
Carbonate mud.
Algal needles.
Mud in suspension.
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EES 450: Sedimentary Geology
CARBONATE MATRIX
• Formation of carbonate mud (micrite).
Carbonate mud Lithification Calcilutite Example thin section
Carbonate muds
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EES 450: Sedimentary Geology
CARBONATE MATRIX
• Formation of carbonate mud (micrite).
- Disintegration of green and brown algae – very important.
- Direct chemical precipitation.
- Bacterial activity and decomposition of organic substances.
- Disintegration of invertebrate shell material and cements through mechanical and biological activity.
- Bioerosion of invertebrate shells and bedrock.
Examples of bioerosion.
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EES 450: Sedimentary Geology
CARBONATE MATRIX
• Destruction or neomorphism of micrite.
- Neomorphism: Recrystallization that changes the size (+,-) and form of crystals in a sedimentary rock. In carbonates this can change a micritic matrix into onedominanted by microspar. Can reduce the aragonite or Mg-rich calcite in a carbonate by replacing them with normal calcite.
Pseudospar (16-50 µm).
Microspar (5-15 µm). Micrite (1-4 µm).
Example of aggrading neomorphism.
CARBONATE MATRIX
• Cement is the chemically precipitated mineral matter that fills the spaces between grains of a sedimentary rock and binds the grains together in a rigid, coherent mass.
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EES 450: Sedimentary Geology
Grains Cement
CARBONATE MATRIX
• Cementation is a diagenetic process.
- Meteoric – H2O of atmospheric origin.- Phreatic – H2O below the water table.- Vadose – H2O above the water table.- Mixed – Mixing of marine and meteoric phreatic H2O.
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EES 450: Sedimentary Geology
CARBONATE MATRIX
• Needed for cementation:
- Large input of CaCO3.- Efficient fluid flow.
• Sources of CaCO3:
- Seawater.- In meteoric and burial settings, dissolution of carbonate sediment.
• Marine cements:
- Mostly high Mg calcite and aragonite.- Cements in order of decreasing solubility:High Mg calcite > aragonite > low Mg calcite > calcite (most soluble varieties are the most common, calcite typically does not precipitate as Mg2+ is abundant in seawater).
• Meteoric diagenesis:
- Meteoric fresh water is almost always undersaturated with respect to Mg2+.- Hence, when marine carbonates come in contact with meteoric water, they are unstable and alter to low Mg calcite as Mg2+ is flushed out of the system. 9
EES 450: Sedimentary Geology
CARBONATE MATRIX
• Major cement types:
- Cement A: Early diagenetic or penecontemporaneous.- Cement B: Late diagenetic formed during sediment consolidation and compaction – cement may be derived from dissolution or pressure solution.
• Cement A:
- Fibrous or bladed cements (vadose, marine phreatic, or submarine).- Usually isopachous, meniscus or stalactitic.
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EES 450: Sedimentary Geology
Isopachous cement of prismatic calcite.
Carbonate thin section – meniscus cement.
Stalactitic cement – Triassic-Jurassic Lias Fm., Atlas Mountains, Morocco.
CARBONATE MATRIX
• Major cement types:
• Cement B:
- Usually involves fresh water (carbonate from dissolution of previous carbonates).- Granular or blocky cements (usually after cement A and initial compaction and cementation).- Drusy-crystal sizes increase from pore walls to center of cavity.- Syntaxial-rim cement – cement in optical continuity with grains (often echinoderm grains).
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EES 450: Sedimentary Geology
Oolitic grainstone, Jurassic age, Morocco – blocky calcite
cement (C).
Syntaxial cement nucleating on echinoid bioclasts (beige).
Syntaxial rim cement.
CARBONATE MATRIX
• Typical cement geometries:
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EES 450: Sedimentary Geology
Blocky, equant Microspar
Bladed Fibrous
Drusy
CARBONATE MATRIX
• Typical cement geometries:
Folk classification of cements 13
EES 450: Sedimentary Geology
CARBONATE MATRIX
• Typical cement geometries:
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EES 450: Sedimentary Geology
Cement A
Cement B
CARBONATE MATRIX
• Multiple geometries:
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EES 450: Sedimentary Geology
