Geology 103 Lecture #19
Carbonate components and classification Reading: Boggs, 5th edition, pp. 135-147
I) Components of carbonate rocks
- The two major classifications systems (Folk and Dunham) use carbonate components to classify or name the rock
A) Allochems (grains)
See Figure 6. 2 from Boggs, Principles of Sedimentology and Stratigraphy, 5th ed., p. 139
- Are often fossil components
- Fossil types may be named individually (by phylum, class or order)
- Other inorganic grains are also common (see section III B below)
- Grains help determine properties of the depositional environment:
- Water depth
- Energy level
- Community structure
B) Micrite (lime mud)- Sample C
- Forms as small needles (< 30 μm)
- From breakdown of aragonitic shells, algae
- Looks gray, dull in hand specimen
- Looks gray or brown, granular in thin section
C) Cement- Sample 22
See Fig. 10.24 from Boggs, Petrology of Sedimentary Rocks (1992) p. 444
- Can be fibrous, bladed, stubby crystals, or optically continuous
- Called “Sparry calcite”
- Cement morphology may tell fresh water vs. marine origin for cementation
- Cement glitters in hand specimen
II) Carbonate classification
- We will use 2 systems:
- Dunham's system: is a good field classification system
- Relies on texture, doesn't identify grains
- Folk's system: a good lab system:
- May need microscopic grain I.D.
A) Dunham's system
See Table 6.3 from Boggs, Principles of Sedimentology and Stratigraphy, 5th edition, p. 146-
- Is based on texture
- Looks at the abundance of grains vs. micrite
- Does not consider the types of grains present
- Terms:
1) Mudstone-
- Is mud (micrite) supported
- Micrite = microcrystalline calcite
- < 10% grains
2) Wackestone-
- Has >10% grains, is still mud supported
3) Packstone-
- Is grain supported
- Has some mud
4) Grainstone-
- Is grain supported
- No lime mud present
5) Boundstone-
- Original components bound together during deposition: by biological activity
- Ex: reef
other terms (added later):
6) Floatstone, Rudstone, Framestone, Bindstone, Bafflestone
B) Folk's system
See Figure 6.9 from Boggs, Principles of Sedimentology and Stratigraphy, 5th edition, p. 145
- The most commonly used carbonate classification system
- Lists the types of grains present
- Describes the proportion of micrite (lime mud) and cement
1) Allochems
a) Ooids-
- Are grains with concentric rings of calcite
- < 2 mm
- Inorganic origin: pick up rings of calcite rolling around in a shallow, high energy marine environment
- note: “oolite” = a rock made of ooids. This is an informal term
- Often have radial structure (in thin section)
b) Bioclasts-
- Are fossil fragments
- Structure, shape vary widely
- Depends on wall structure of the original fossil shell
c) Peloids
- Are often fecal pellets
- Are lumps of micrite (lime mud) = granular
d) Intraclasts-
- The equivalent of rip-up clasts
- Muddy pieces of ocean floor- usually lime mud
See Figure 6. 2 from Boggs, Principles of Sedimentology and Stratigraphy, 5th ed., p. 139
2) Micrite vs. (sparry) calcite cement
- Rock names have a suffix:
See Figure 6.8 from Boggs, Principles of Sedimentology and Stratigraphy, 5th edition, p. 145
a) Micrite
- Lime mud dominates
- Micrite = microcrystalline calcite
- < 30 μm
- Indicates quiet water environment
- Note: Dismicrite is a descriptive term that Folk’s uses when the fine lime mud begins to recrystallize. This usually occurs as small patches of calcite crystals in a rock that is dominantly micrite (lime mud). Crystals are very small (mm scale), and are a minor part of the rock.
b) Sparite
- Sparry calcite dominates
- Sparry calcite is coarser cement
- Millimeters (mm) scale
- Crystals form between grains
- Cement forms after deposition
- Cementation is relatively rapid: may be 1000’s of years
-Names can be used in combination, with most abundant grain first:
Ex: oobiopelsparite- has ooids, bioclasts, and peliods
- Many carbonate people use Dunham’s textures, Folks grain types:
Ex: oobiopelpackstone