Limestone
There are two primary systems for classifying Limestone, The Folk, and the Dunham classification. In mudlogging the Dunham classification is better suited when writing descriptions because it speaks about the depositional properties whereas the Folk classification talks about the textural parameters and implied depositional conditions. As mud loggers our primary duty is to Identify the properties of the cuttings not to analyze the basin. So, for our purposes we will be using the Dunham classification.
In 1962 Robert J. Dunham published his classification system for limestone. The original system was developed to provide convenient depositional texture-based classification that focuses attention on the textural properties for interpreting the depositional environment of the rocks.
The three criteria used to define the original Dunham classes were:
- The supporting fabric of the original sediment.
- The presence or absence of mud (the fraction <20 μm in size) (clay sized particles).
- Evidence that the sediments were organically bound at the time of deposition.
Originally, he came up with four classes based on the above criteria.
Those classes were:
- Mudstone: A mud-supported carbonate rock containing <10% grains.
- Wackestone: A mud-supported carbonate lithology containing >10% grains.
- Packstone: A grain-supported fabric containing 1% or more mud-grade fraction.
- Grainstone: A grain-supported carbonate rock with <1% mud.
Dunham later defined two additional classes when he recognized that these classes did not encompass all carbonate lithologies.
Those other classes were:
- Boundstone: where there is any evidence that the carbonate sediments were bound at the time of deposition.
- Crystalline dolomite or Crystalline limestone: where recrystallization has resulted in the original depositional fabric of a carbonate rock cannot be identified.
Dunham specifically stated that, where appropriate, these six textural class names are intended to be combined with modifiers describing grains and mineralogy.
The original classification can be summarized as follows:
