What about rocks that do not readily fit into either the Folk or Dunham
limestone classification, nor the siliciclastic rock classification as described
in the companion Sandstone Tutorial?
For example, there are rocks with
abundant skeletal allochems or other grains of intrabasinal origin (phosphate,
glauconite) in a siliciclastic or mixed siliciclastic/carbonate matrix. One
solution is to classify rocks as limestones if they contain >50% carbonate
minerals by weight or volume and otherwise to classify them as sandstones
(classified on extrabasinal grain types) or siliciclastic mudstones (generally
classified on texture alone). Even so, neither the Folk nor Dunham
classifications strictly accommodate rocks for which the detrital matrix is not
carbonate mud, but rather siliciclastic or mixed siliciclastic/carbonate
material of silt- and clay-size. It is easy to envision a rock composed of 40%
oyster shells in a matrix composed of 20% carbonate mud mixed with 20% clay
minerals and 20% quartz silt. The whole rock is composed of 60% carbonate, but
the interstitial mud between the mollusks contains only one third carbonate.
"Micrite" or "carbonate mud" does not nearly describe the interstitial
fine-grained material, so can we still say "mollusk packstone"? This is, in
fact, the approach used in this tutorial, with the added descriptor "clayey" to
signify the clay mineral content. The inverse complication occurs when skeletal
debris of non-carbonate composition is abundant in either a carbonate- or
siliciclastic-rich matrix. Resorting to "packstone" or "biomicrite" in such a
case seems a more suitable solution than relying on a siliciclastic
classification scheme that ignores the abundant fossils altogether or refers to
them only by their particle size.
Dolomites are another case for which the Folk and Dunham approaches are sometimes less
than satisfactory. Labeling a rock a "finely crystalline dolomite" is not a good solution
if clear evidence of the rock's primary texture has survived the dolomitization process.
In such cases of extensive replacement with textural preservation, "dolomitized" or
"silicified" are examples of descriptors that can be added to the Folk or Dunham
name.