There are, like in sandstone, dozens of classifications for the carbonate rocks. It is not necessary to learn then all. Here you will explore first the general terminology one may use within the field environment where one only has a hand lens, rock hammer, and possibly a bottle of dilute hydrochloric acid. Then we will examine a classification scheme that can, with some difficulty, be applied in the field, but which is better suited for lab examination of polished slabs. Finally you will explore a classification which requires the application of the petrographic microscope.
Two primary terms apply here limestone and dolostone. If you think the rock is at least 50% calcite it is limestone and if you think that it is at least 50% dolomite call it a dolostone (some folks still call such a dolomite but get used to using dolostone instead for the rock name). To tell the two apart you can use the acid test you learned in Physical Geology....calcite fizzes readily and dolomite not so readily. But keep in mind on hot rock dolomite will fizz like crazy and cold dense calcite will react like dolomite. Also it is important to use a fresh surface in order to avoid having the caliche crust (calcite) on dolostone making you think that you have a limestone at hand. (What is caliche? How does caliche form? HOMEWORK!!!!) Another problem you may encounter is that a cold dolostone with a sucrossic (sugar like) texture will not fizz and will look to you just like sandstone. Remember quartz will scratch the steel or your pocketknife or hammer and dolomite will not. Use of a hand lens will usually help to solve this. A past director of the United States Geological Survey had a problem with telling sucrossic dolostone from a buff colored sandstone.
You can use the stain Alizarin red-S in the field, it really works quite nicely. Take a fresh caliche free surface, wash it with the dye mixture and then flush it gently with clean water. Examine the stained surface with your hand lens. Calcite will be pink and dolomite will be clear just like in thin sections. Do not trust your observations after the surface dries. The potassium ferricyanide blue stain does not work well here though some workers claim to have had good results.
You may apply to the term limestone or dolostone a number of adjectives to more completely name the rock:
Dolomitic
Calcareous
Argillaceous
Arenaceous
Fossiliferous
Oolitic
Pisolitic:
Cherty
Brecciated
There are other terms, read the Glossary on the next rainy day!!
The full field description of carbonate rocks follows the same procedures discussed in the rock description lab.
If you are back in the office and have at hand a cut slab you can observe detail that you would not generally see in the field. The slab could be highly polished but if it is not just wet it with water. Staining it with alizarin red S would be useful. Dunhams (1962) classification works real well here for putting a name on the rock. You have seven basic terms to use plus a few later modifications from Dunham’s original work.
Mudstone: the rock is as you see it mostly made of carbonate mud or cryptocrystalline carbonate matrix. Grains (fossils, ooids, etc.) will be less than 10 % of the rock. Sometimes you will be wrong in your call here; sometimes cloudy calcite spar (crystals) looks like carbonate mud other times mud looks like spar...but call it as you see it.
Wackestone: grains make up more than 10% of the rock but the grains are "mud supported" they float in the mud matrix.
Packstone: Lots of sandbox sized grains with mud between them but the grains are grain supported.
Grainstone: Sandbox sized grains with spar between them, little or no mud.
Floatstone: 10% or more of the grains are greater than 2mm in diameter and mud in the matrix (like a packstone)
Rudstone: 10% or more of the grains are greater than 2 mm in diameter and spar is between the grains (like grainstone).
Boundstone: "Original components organically bound during deposition" think of colonial corals and stromatolites
Embry and Klovan (1971) working in Devonian reefs of British Columbia added three subdivisions to the Boundstones. These are now widely used as a modification to Dunham.
Bafflestone: "organism acted as baffles"
Bindstone: "organisms encrusting and binding"
Framestone "organisms building a rigid framework"
The foundation of Folk’s classification is the relative amounts of (1) allochems, (2) calcite cement or 'spar", and (3) microcrystalline to cryptocrystalline calcite matrix or 'micrite. These amounts are best ascertained with the petrographic microscope and generally with the application of modal analysis but with some skill and practice satisfactory results can be obtained with good quality hand samples. Allochems are sepatated into four types, intraclasts, ooids, pellets, and bioclasts.
Think of intraclasts as being 'intra-formational rock fragments'. They can be of any size but generally will range above 2 mm in diameter and less than a few centimeters in diameter. Compositionally they usually consist of microcrystalline to cryptocrystalline calcite but they can also contain within them other allochems such as small bioclasts like ostracods. Their shape is variable and frequently they will be rounded.
Ooids are carbonate grains which are usually small less than 1 mm in diameter spherical grains which posses a series of concentric laminations when view in cross section. One must recognize the concentric laminations in order to apply the ooid name. Those ooids, which have diameters greater than a few millimeters, are generally referred to by carbonate petrologists as pisolites.
Pellets are microcrystalline to cryptocrystalline calcite or aragonite grains that are probably of fecal origins. They are in most cases less than 2 mm in maximum diameter and elongate or roller in external shape.
Bioclasts are the calcite or aragonite tests of organisms that lived contemporaneously with the depositional processes which formed the sediment. There are a wide variety of types of bioclasts each with its own unique texture and shape. Excluded from the bioclasts are any fossils that are not carbonate in nature, teeth, bones, and conodonts. Also excluded are any bioclasts, which were 'fossils' at the time of deposition. Theses would be considered to be detritus or rock fragments (sedimentary rock fragments SRF or Folk's sandstone classification).
In practice some problems result from the above-idealized characterizations of these allochems. Early (pre-depositional to syn-depositional) diagenetic processes can render ooids and bioclasts indistinguishable from intraclasts. This is called micritization. Furthermore it is almost impossible to say with confidence that a pellet is of fecal origins unless one witnessed the event. It could just as easily have been an intraclast. For these reasons carbonate petrologists usually when working with Folk's classification call any microcrocrystalline to cryptocrystalline carbonate grain under 2 mm in diameter a pelloid and treat pelloid as being synonymous with Folk's pellet. Similarly any microcrystalline to cryptocrystalline carbonat grain over 2mm in diameter and regardless of its true origins is referred to as an intraclasts.
Folk' scheme consists of five classes of carbonate rocks which are designated in geologic shorthand by the Roman numerals I, II, III, IV, V. Class I and II are limestones (calcite and aragonite rocks), partially dolomitized limestones (dolomite <50%), and primary dolomites (their existance is of debate) which have greater than 10% by volume carbonate allochems. Class I rocks have mesocrystaline (easy to see individual crystals) calcite know as spar between the allochems. The Class II rocks have micrite (microcrystalline to crypocrystalline calcite) filling the space between the allochems. Class III rocks all have less than 10% by volume carbonate allochems. Class IV rocks are all "undisturbed bioherm rocks" which are almost synonymous with Dunham's boundstones. The 'bafflestone' of Kloven and Emory's modified Dunham's classification does not fit well in this class. Class V rocks consists entirely of replacement dolomite.
Class I and II rocks are further subdivided by the relative percentage of the four different types of allochems and the size range of the allochems. Dealing with size of the allochem first, if the population of allochems is generally greater than 2 mm in diameter than the root name follows that of Grabau or rudite while if they are less than 2 mm in diameter the root name follows the Grabau term arenite. However in the actual name giving the 'aren' portion of arenite is deleated. Class I rocks have spar between the allochems and hence are named the sparites and sparrudite. Class I rocks have micrite between the allochems and are thus named the micrites and micrudites.
Each of these four terms must have a prefix that denotes the actual allochem content of the rock. If more than 25% by volume of the allochems (not the rock) are intraclasts then the prefix 'intra' is applied as in intrasparite, intrasparrudite, intramicrite, and intramicrudite. In the pure sense of folk's classification all four rocks could exists however in practice since one is forced to attributes all microcrystalline to cryptocrystalline calcite grains under 2 mm in diameter to the pellets or pelloids the terms intrasparite and intramicrite are not used.
If the rock has 25% or less intraclasts but more than 25% by volume ooids the prefix 'oo' is applied as in oosparite, oosparrudite, oomicrite, oomicrudite. Since ooids greater then 2 mm are uncommon and generally called pisolites the terms oosparrudite and oomicrudite are rarely used. Furthermore an oomicrite is itself a rare rock type as the process that cause the formation of ooids generally excludes the formation of a lime mud or microcrystalline to cryptocrystalline calcite matrix.
If the rock has 25% or less intraclasts, 25% or less ooids, and 67% or greater bioclasts that the prefix 'bio' is applied as in biosparite, biosparrudite, biomicrite, biomicrudite. If the rock has 25% or less intraclasts, 25% or less ooids, and 67% or more pellets (pelloids) the prefix 'pel' is applied as in pelsparite, pelsparrudite, pelmicrite, and pelmicrudite. However, since in practice pellets over 2mm in diameter were taken with the intraclasts, one would not have use for the names pelsparrudite and pelmicrudite. If the rock has 25% or less intraclasts, 25% or less ooids, and has pellets and bioclasts in roughly the same amounts (33% to 67%) the prefix 'biopel' is applied as in biopelsparite, biopelsparrudite, biopelmicrite, and biopelmicrudite. The 'bio' always proceeds the 'pel' regardless of which is the most abundant.
The class III rocks contain no more than 10% by volume allochems the remainder of their volume being mostly micrite (microcrystalline to cryptocrystalline calcite) hence these are the micrites. If there allochems and the dominant allochems are intraclasts the rock is called an intraclast-bearing micrite. If the dominant allochems are ooids it is an oolite-bearing micrite. If bioclasts are the dominate allochem then the rock is a fossiliferous micrite. If pellets (pelloids) dominate then pelletiferous micrite. If there is less than 1% allochems in the rock it is simply a micrite. Some micrite (rock name) will have patchy areas of calcite spar. These special micrites are called dismicrite. And if the rock is composed of microcrystalline to cryptocrystaline dolomite Folk applies the name dolomicrite. This latter name is now in disuse in favor of aphanocyrstalline dolomite, a rock which falls under Folk's class V.
The class IV rocks are the biolithites and they lack further subdivision though such would be useful. If one is dealing with biolithites it would be advisable to switch to Dunham's classification. The bafflestones of the modified Dunham classification do not fit into Folk's biolithites very well and in most cases the terms biomicrudite or fossiliferous micrite would be applied to them.
Class V rocks are the 'replacement dolomites'. Which means that the rock is mostly dolomite and that you have applied alizarin red-s to the rock to verify this. All class V rocks are called dolomite by Folk or dolostone by more recent workers. All have an adjective placed before the name that expresses the typical size range of the dolomite crystals.
|
Aphanocrystalline |
Under 0.0039 mm |
|
Very finely crystalline |
0.0039 to 0.0156 mm |
|
Finely crystalline |
0.0156 to 0.0625 mm |
|
Medium crystalline |
0.0625 to 0.25 mm |
|
Coarsely crystalline |
0.25 to 1.00 mm |
|
Very coarsely crystalline |
1.00 to 4.00 mm |
|
Extremely coarsely crystalline |
Over 4.00 mm |
If the rock contains what are called 'allochem ghosts' then an additional adjective is applied depending upon the dominant allochem ghost present; intraclastic, oolitic, biogenic, or pellet. These ghosts are the images of the original allochems that have now become replaced by the secondary mineral dolomite. This will become clearer in a later lab exercise.
If the rock of any class is observed to have 10% or more quartz sand, silt or clay as terrugenous detritus grains then the adjectives areanaceous, silty, or argillaceous are applied respectively. Likewise significant amounts (10% or more) of glauconite or phospohorite will yield a related adjective (note these are primary components of the sediment and not authigenetic components).
| Biomicrite | |
| Biosparite | |
| Biopelsparite | |
| Intrasparrudinte | |
| Pelsparrite | |
| Biolithite (stromatoporoid) | |
Exercise#1: For the specified group of hand samples supply a Dunham classification name.
Exercise #2: For the specified group of thin sections perform a 100 hit modal analysis and determine a Folk classification name.
Folk, R.L., 1959. Practical petrographic classification of limestones. Bulletin American Association Petroleum Geologists, v. 43, p. 1-38
Folk, R. L., 1962. Spectral subdivisions of limestone types. In: Ham (ed) Classification of Carbonate Rocks. Memoir 1, American Association of Petroleum Geologists, p. 62-85.
Strohmenger, C. and Wirsing, G., 1991. A proposed extension of Folk's (1959, 1962) textural classification of carbonate rocks. Carbonate and Evaporites, v. 6, p. 23-28.
Dunham, R, J., 1962. Classification of carbonate rocks according to depositional texture. In: Ham (ed) Classification of Carbonate Rocks. Memoir 1, American Association of Petroleum Geologists, p. 108-121.
Embry, A.F. and Klovan, J.E., 1971. A late Devonian reef tract on northeastern Banks Island Northwest Territories. Bulletin Canadian Petroleum Geologists, v. 19, p. 730-781. (Examine the part dealing with modification of Dunham's classification).
Carozzi, A. V., 1989. Carbonate rock depositional Models. A microfacies approach. Prentice Hall, Englewood Cliffs, N.J. Advanced Reference Series, 604 p.
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Last modified 10/11/2005