Carbonate Identification and Classification
Two carbonate classification
systems are in common use today, one by R.L. Folk and the second by R.J.
Dunham . The Dunham system is based on depositional texture (that is, the
amount of matrix surrounding the grains at the time of deposition). It
uses such names as mudstone, wackestone, packstone, grainstone, and
boundstone. Each classification has its strengths, but we use the Folk
system is illustrated here.
Micrite is "lime mud", the dense, dull-looking sediment made of clay sized
crystals of CaCO3. Much micrite today forms from the breakdown
of calcareous algae skeletons. It is not clear if all ancient micrites
formed in the same way. Many carbonates are composed of nearly 100%
micrite. Such rocks are simply called
But what happens if there is more than one allochem in the rock, or there is a mixture of micrite and spar? This classification system has great flexibility and creativity. You can easily build your own descriptive rock names. The name is built up by stringing together all the allochem names in order from least to most abundant, and then adding the interstitial material name ("matrix" below for short). For example, a rock like this:
Oolites + Fossils + Spar matrix = Oo bio sparite
The name is written as one word, Oobiosparite.
Another example (again allochems from least to most abundant):
Pellets + Oolites + Fossils + Micrite matrix = pel oo bio micrite
The name is written as one word, Peloobiomicrite.
But what if there is both micrite and spar matrix? The system is the same; just list them from least to most abundant.
Fossils + Spar matrix + Micrite matrix = bio spar micrite
This system goves through other levels of refinement, such as the table below where the abundance of allochems is dealt with. Other modifiers deal with different sizes of allochems. But all this is beyond our discussion here.
A classification such as this one works well if you want to construct rock names from observations. The system, however, does not lend itself well to constructing keys for classification. A key requires the establishment of arbitrary categories of rocks, and a system like the one above deal with all the myriad combinations that are possible.
Nonetheless, we have constructed a key to identify rocks based on their allochems and interstitial material. Just be aware that its main weakness is that there are always rocks that do not fit easily into its simple categories.
Carbonate Identification Key
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