SPRING 2013GEOLOGY 285:INTRO. PETROLOGYDr. Helen LangDept. of Geology & GeographyWest Virginia UniversityLimestones and DolomitesCarbonate rocks, usually deposited by or with the help of biological organismsTheir mineralogy is simple• Carbonate minerals:– Calcite CaCO3– Aragonite CaCO3– Dolomite CaMg(CO3)2• Minor Quartz and/or ClayPolymorphsParticle Types and Textures are Important for Classification• Allochemical Particles (allochems)– framework grains of a mechanically deposited limestone– four main types– some formed of calcite, some aragonite• Orthochemical Particles (orthochems)– matrix and cement that fill spaces, bind allochems together and lithify the sedimentAllochems• Fossils - solid carbonate remains of organisms (fossils), and fragments of fossils• Peloids - ellipsoidal aggregates of micro-crystalline CaCO3, lack internal structure, mostly fecal pellets of worms, fish, etc.• Oöliths - spherical polycrystalline carbonate particles of sand size, with concentric or radial internal structure, commonly have a nucleus for precipitation• Limeclasts - fragments of earlier-formed limestone, mostly intraclasts from a local sourceFossilsWith Quartz silt Mostly castsNote Calcite stains redPeloidsare structureless, composed mostly of micriteOöidsHave radial and/or concentric internal structuresOrthochems• Microcrystalline Calcite (Micrite) - CaCO3mud, disarticulated algal material, carbonate ooze, ≤ 4 m diameter [Note difference between lime mud and silicate mud]• Coarsely crystalline calcite (Sparry calcite or “Sparite”) - calcite cement, precipitated from pore fluid (inorganic ppt.)• Usually one or the other, not bothMicrite with a few FossilsCarbonate mud matrixSparite=coarse crystalline calcite (Cc is stained pink)Non-carbonate Minerals• Typically less than 5% terrigenous detritus (quartz, clay, chert)• Limestones form only where input of terrigenous detritus (especially mud) is minimal: fresh water changes salinity, organisms are killed or buried by mud• Chert is intrabasinal from siliceous organisms or is diageneticGulf of MexicoLimestones occur where clastic input is minimalThere are two commonly used limestone classification schemes• Folk’s Classification– Based on major allochem and major orthochem– Hard to use without thin sections– Not as useful in the field• Dunham’s Classification– Based on structure and percent grains vs. mud– More useful in the fieldFolk’s Classification• Major Allochem (prefix)• Fossils bio-• Peloids pel-• Oöliths oö-• Limeclasts intra-• Major Orthochem (suffix)• -micrite• -sparite• >90% micrite is just called “micrite”BiomicriteFolk Name?OospariteFolk Name?Folk Name? MicriteDunham: field classificationUse names of fossils or other allochems as modifiersDunham: field classificationUse names of fossils or other allochems as
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