Dr. Helen Lang
Dept.
p of Geology
gy & Geography
g p y
West Virginia University
SPRING 2012
GEOLOGY 285:
INTRO. PETROLOGY
Limestones and Dolomites
Carbonate rocks, usually deposited by or
p of biological
g
organisms
g
with the help
Their mineralogy is simple
• Carbonate minerals:
– Calcite
– Aragonite
– Dolomite
CaCO3
Polymorphs
CaCO3
CaMg(CO3)2
• Minor Quartz and/or Clay
Particle Types and Textures are
Important for Classification
• Allochemical Particles (allochems)
– framework grains of a mechanically deposited
limestone
– four main types
yp
– some formed of calcite, some aragonite
• Orthochemical Particles (orthochems)
– matrix and cement that fill spaces, bind
allochems together and lithify the sediment
Allochems
• Fossils - solid carbonate remains of organisms
(fossils) and fragments of fossils
(fossils),
• Peloids - ellipsoidal aggregates of micro-crystalline
CaCO3, lack internal structure,
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
With Quartz silt
Mostly casts
N C
Note
Calcite
l i stains
i redd
Peloids
are structureless, composed mostly of micrite
Oöids
Have radial and/or
concentric internal
structures
Orthochems
• Microcrystalline Calcite (Micrite) - CaCO3
mud, disarticulated algal material, carbonate
ooze, ≤ 4 μm diameter [Note difference
between lime mud and silicate mud]
• Coarsely crystalline calcite (Sparry calcite
or “Sparite”) - calcite cement, precipitated
from pore fluid (inorganic ppt.)
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,
(q
y, 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
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
B d on structure
t t
andd percentt grains
i vs. mudd
– More useful in the field
Folk’s Classification
• Major Allochem
(prefix)
• Fossils
bio• Peloids
pel• Oöliths
oö
oö• Limeclasts
intra-
• Major Orthochem
(suffix)
• -micrite
• -sparite
• >90% micrite is just
called
ll d “micrite”
“ i it ”
Folk Name?
Biomicrite
Folk Name?
Oosparite
Folk Name?
Micrite
Dunham: field classification
Use names of fossils or other allochems as modifiers
Dunham: field classification
Use names of fossils or other allochems as modifiers