geology 285 - università di bari - home 1.pdf · limestone classification schemes • folk’s...
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Dr. Helen LangDept. of Geology & Geographyp gy g p yWest Virginia University
SPRING 2012
GEOLOGY 285:INTRO. PETROLOGYINTRO. PETROLOGY
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Limestones and DolomitesLimestones and Dolomites
Carbonate rocks, usually deposited by or with the help of biological organismsp g g
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Their mineralogy is simple
• Carbonate minerals:– Calcite CaCO3Calcite CaCO3
– Aragonite CaCO3
– Dolomite CaMg(CO3)2
Polymorphs
Dolomite CaMg(CO3)2
• Minor Quartz and/or Clay
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Particle Types and Textures areParticle Types and Textures are Important for Classification
• Allochemical Particles (allochems)– framework grains of a mechanically depositedframework grains of a mechanically deposited
limestone– four main typesyp– some formed of calcite, some aragonite
• Orthochemical Particles (orthochems)Orthochemical Particles (orthochems)– matrix and cement that fill spaces, bind
allochems together and lithify the sedimentallochems together and lithify the sediment
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Allochems• Fossils - solid carbonate remains of organisms
(fossils) and fragments of fossils(fossils), and fragments of fossils• Peloids - ellipsoidal aggregates of micro-crystalline
CaCO lack internal structure mostly fecal pellets ofCaCO3, lack internal structure, mostly fecal pellets of worms, fish, etc.
• Oöliths spherical polycrystalline carbonate particles• Oöliths - spherical polycrystalline carbonate particles of sand size, with concentric or radial internal structure, commonly have a nucleus for precipitationcommonly have a nucleus for precipitation
• Limeclasts - fragments of earlier-formed limestone, mostly intraclasts from a local sourcemostly intraclasts from a local source
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Fossils
With Quartz silt Mostly casts
N C l i i dNote Calcite stains red
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Peloids
are structureless, composed mostly of micrite
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OöidsOöids
Have radial and/or concentric internal structures
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OrthochemsOrthochems• Microcrystalline Calcite (Micrite) - CaCO3Microcrystalline Calcite (Micrite) CaCO3
mud, disarticulated algal material, carbonate ooze, ≤ 4 μm diameter [Note differenceooze, ≤ 4 μm diameter [Note difference between lime mud and silicate mud]
• Coarsely crystalline calcite (Sparry calcite• Coarsely crystalline calcite (Sparry calcite or “Sparite”) - calcite cement, precipitated from pore fluid (inorganic ppt )from pore fluid (inorganic ppt.)
• Usually one or the other, not both
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Micrite with a few Fossils
Carbonate mud matrix
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Sparite=coarse crystalline calcite (Cc is stained pink)
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Non-carbonate Minerals
• Typically less than 5% terrigenous detritus (quartz, clay, chert)(q , y, )
• Limestones form only where input of terrigenous detritus (especially mud) isterrigenous detritus (especially mud) is minimal: fresh water changes salinity, organisms are killed or buried by mudorganisms are killed or buried by mud
• Chert is intrabasinal from siliceous organisms or is diageneticorganisms or is diagenetic
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Gulf of MexicoLimestones occur where clastic input is minimal
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There are two commonly used limestone classification schemes
• Folk’s Classification– Based on major allochem and major orthochemBased on major allochem and major orthochem– Hard to use without thin sections– Not as useful in the fieldNot as useful in the field
• Dunham’s ClassificationB d t t d t i d– Based on structure and percent grains vs. mud
– More useful in the field
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Folk’s Classification
• Major Allochem • Major Orthochem (prefix)
• Fossils bio-(suffix)
• -micrite• Peloids pel-• Oöliths oö
• -sparite• Oöliths oö-• Limeclasts intra- • >90% micrite is just
ll d “ i it ”called “micrite”
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BiomicriteFolk Name?
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OospariteFolk Name?
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Folk Name? MicriteFolk Name? Micrite
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Dunham: field classification
Use names of fossils or other allochems as modifiersUse names of fossils or other allochems as modifiers
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Dunham: field classification
Use names of fossils or other allochems as modifiersUse names of fossils or other allochems as modifiers