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Non-Terrigenous Sediments and Rocks

Carbonate Sediments and Rocks

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No “Simple” Classification Scheme

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Most Common non-Terrigenous Sedimentary Rocks• Carbonates (>50% primary carbonate minerals)

– Limestone (CaCO3)• Chemical• biochemical

– Dolomite (CaMg(CO3)2)• Chemical

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Carbonate Rock Classification

• Allochems– Carbonate sand

• Micrite– Micro Crystalline calcite

• Cement (spar)

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Carbonate Rocks Constituents• The sand-sized grains that occur in carbonate rocks are called

allochemical particles or allochems.

1. Intraclasts

2. Ooliths

3. Peloids

4. Skeletal particles (bioclasts)

• The interpretation of the depositional setting of carbonates is based on grain types, grain packing or fabric, sedimentary structures, and early diagenetic changes.

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Carbonate Rocks Constituents

• The sand-sized grains that occur in carbonate rocks are called allochemical particles or allochems.

1. Intraclasts (rock fragments):• formed, transported and redeposited within the basin

2. Ooliths: concentrically laminated carbonate structures, including: • oolites -concentrically laminated structures,less than 2mm in diameter, thought

to be abiogenic in origin • pisolites - same as oolites, but greater than 2mm in diameter • oncolites - spheroidal stromatolites (> 1-2 cm)

3. Peloids: • silt to fine grained sand sized carbonate particles with no distinctive internal

structure; most thought to be fecal pellets4. Skeletal particles (bioclasts):

• whole microfossils, whole megafossils, broken shell fragments – algae, forams, corals, bryozoans, brachiopods, gastropods, pelecypods,

ostracods, etc.

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Carbonate Rocks Constituents

• Intraclasts (early lithified carbonate fragments):• irregularly-shaped grains that form by syndepositional erosion of

partially lithified sediment.

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Carbonate Rocks Constituents

• Ooliths: concentrically laminated carbonate structures, including:

– oolites -concentrically laminated structures,less than 2mm in diameter, thought to be abiogenic in origin

– pisolites - same as oolites, but greater than 2mm in diameter

– oncolites - spheroidal stromatolites (> 1-2 cm)

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Carbonate Rocks Constituents• Pelloids:

– silt to fine grained sand sized carbonate particles with no distinctive internal structure; most thought to be fecal pellets

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Carbonate Rocks Constituents• Skeletal particles (bioclasts):

– whole microfossils, whole megafossils, broken shell fragments

• algae, forams, corals, bryozoans, brachiopods, gastropods, pelecypods, ostracods, etc.

• Standard microfacies (fossil fragment type -> environment)

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Carbonate Rocks Constituents• Micrite:

– microcrystalline carbonate particles of clay (<1-4 micron) size (subtranslucent matrix) formed by

• chemical or biochemical ppt

• abrasion of allochems

– implies deposition in a low energy environment just like in terrigenous mudstones

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Carbonate Rocks Constituents• Cement:

– sparry (twinkling crystalline) orthochemical material formed in interstitial pore spaces of “grainy” carbonate sediment

• cement in pores indicates original void space

• also recrystallized allochems or micrite

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Carbonate Rocks Constituents• Insoluble Residues –

– minor amounts of clay minerals and quartz occur in limestones, as insoluble residues, (so called because they do not dissolve in HCl)

– Most insoluble material is chert (siliceous)

– chert mostly originates from the shells of silica secreting organisms including diatoms, radiolarians, and some sponges.

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Classification Schemes: Folk Classification

• Type I limestone, – Sparry Allochemical rocks: allochems

> 50%, spar cement > micrite mud [4 rock types]

• more energetic environment, some sorting

• Type II limestone, – Micritic Allochemical rocks:

allochems >10%, micrite mud > spar cement [4 rock types]lower energy environment, more poorly sorted than Type I

• Type III limestone: Micrite: allochems < 10%– very low energy at the site of

deposition (carbonate mudrock)

• “Biolithite”: Reef rock

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Classification Schemes: Dunham Classification

• Dunham Classification– Texture and allochem type

incorporated into classification• sediment deposited in calm vs

agitated waters

• mud-bearing vs mud-free sediment

• grain vs mud support

• bound (biologically)

• depositional texture recognizable

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5 Principles of Dunham Classification1. Presence or absence of lime mud; is there any mud at all.

Calm waters allow for the accumulation of lime mud and indicates the absence of current induced agitation

2. Grain Support: self supporting framework: • fluid circulation, diagenesis

3. Grain kind: standard microfacies types4. Grain size, rounding, and coating: hydrologic interpretations5. Biogenically ppt masses bound at time of deposition:

– Boundstone– organic framework– laminations not consistent with gravity (stromatolite)– roof over sediment filled cavities

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Folk Textural Spectrum Classification

• Concocted to incorporate textural characteristics comparable to textural maturity in TC sediments– Mud component

– Sorting

– Rounding

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Dolomitic Rocks

• Typically devoid of primary textures and structures; if primary textures are preserved– <10% dolomite: “dolomitized” (rock name)– >10% dolomite: dolomitic (rock name)– recrystallized carbonate: dolostone

• saddle dolomite: “burial” dolomite of hydrothermal origin