Geol 285 - Dr. Helen Lang, West Virginia University, Spring 2009
Limestones and Dolomites
Carbonate rocks, usually deposited with the help of biological organisms
Mineralogy is simple

Carbonate minerals:

Calcite
CaCO3

Aragonite
CaCO3

Dolomite
CaMg(CO3)2

Minor Quartz and/or Clay
Therefore, Particle 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 sediment
Allochems

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 source
Fossils - Brachiopod Shell
Fossils
Peloids
Oöids
Orthochems

Microcrystalline Calcite (Micrite) - CaCO3 mud, disarticulated algal material,
carbonate ooze, 1- 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 both
Micrite
Sparite=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 diagenetic
Gulf of Mexico
Limestones occur where clastic input is minimal
There are two commonly used classification schemes
• Folk’s Classification
– Based on major allochem and major orthochem
– Hard to use without thin sections
– Not useful in the field
• Dunham’s Classification
– Based on structure and percent grains vs. mud
– More useful in the field
Folk’s Classification

Major Allochem (prefix) Major Orthochem (suffix)

Fossils

Peloids pel-
bio-
-micrite
-sparite

Ooliths
oo-

Limeclasts
intra-

>90% micrite is just called Micrite
Folk Name? Biomicrite
Folk Name? Oosparite
Folk Name? Micrite
Dunham: field classification (see handout)