Classification of Rocks
Rocks are classified in order to communicate information about them. All classifications of limestones are arbitrary and they frequently overlap or do not fit ones particular needs. Since binocular microscopes or hand lenses are the tools that are commonly available to the explorationist, a practical classification should be based on descriptions using them. When these instruments are used, it is usually possible to identify the individual grains forming the rock. Thus most classifications require that the most significant sedimentary particle in the rock be described. For instance, if a rock is composed of ooids, it is termed and oolitic limestone. If the limestone also contains a minor element such as skeletal fragments, then it is called a skeletaloolitic limestone.
Two of the most widely used classifications are those of Folk (1959,1962) and Dunham (1962). Both classifications subdivide limestones primarily on the basis of matrix content.
Most limestones are classified by Folk allochemical rocks if they contain over l0% allochems (transported carbonate grains). Based on the percentage of interstitial material, the rocks may be further subdivided into two groups: sparry allochemical limestones (containing a sparry calcite cement of clear coarsely crystalline mosaic calcite crystals) and microcrystalline allochemical limestone(containing microcrystalline calcite mud, micrite, which is subtranslucent grayish or brownish particles less than about 5 microns in size). Further subdivision is based on the allochem ratios of Folk (1962) are shown in Scholle & Ulmer-Scholle (2003).
Thus Folk's classification (figures above) is most suited for thin section study. Remember that he terms rocks with appreciable matrix as micrites while matrix-free rocks that contain sparry calcite cement are termed sparites. As you can see sparites and micrites are further subdivided by means of their most common grains.
In contrast, Dunham's classification (figures
Two of the most widely used classifications are those of Folk (1959,1962) and Dunham (1962). Both classifications subdivide limestones primarily on the basis of matrix content.
Most limestones are classified by Folk allochemical rocks if they contain over l0% allochems (transported carbonate grains). Based on the percentage of interstitial material, the rocks may be further subdivided into two groups: sparry allochemical limestones (containing a sparry calcite cement of clear coarsely crystalline mosaic calcite crystals) and microcrystalline allochemical limestone(containing microcrystalline calcite mud, micrite, which is subtranslucent grayish or brownish particles less than about 5 microns in size). Further subdivision is based on the allochem ratios of Folk (1962) are shown in Scholle & Ulmer-Scholle (2003).
Thus Folk's classification (figures above) is most suited for thin section study. Remember that he terms rocks with appreciable matrix as micrites while matrix-free rocks that contain sparry calcite cement are termed sparites. As you can see sparites and micrites are further subdivided by means of their most common grains.
In contrast, Dunham's classification (figures