Dr. Helen LangDept. of Geology & Geographyp gy g p yWest Virginia University

SPRING 2012

GEOLOGY 285:INTRO. PETROLOGYINTRO. PETROLOGY

Limestones and DolomitesLimestones and Dolomites

Carbonate rocks, usually deposited by or with the help of biological organismsp g g

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

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

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

Fossils

With Quartz silt Mostly casts

N C l i i dNote Calcite stains red

Peloids

are structureless, composed mostly of micrite

OöidsOöids

Have radial and/or concentric internal structures

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

Micrite with a few Fossils

Carbonate mud matrix

Sparite=coarse crystalline calcite (Cc is stained pink)

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

Gulf of MexicoLimestones occur where clastic input is minimal

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

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”

BiomicriteFolk Name?

OospariteFolk Name?

Folk Name? MicriteFolk Name? Micrite

Dunham: field classification

Use names of fossils or other allochems as modifiersUse names of fossils or other allochems as modifiers

Dunham: field classification

Use names of fossils or other allochems as modifiersUse names of fossils or other allochems as modifiers