Friday, January 9, 2009

Limestone classification - naming your rock

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Something I should be doing a lot now is giving names to rocks I find in the field. Unfortunately a white, powdery substance also known as snow is preventing this. I figured I might share with you the joys of limestone classification. Perhaps it may help you understand my posts or pass that little exam ahead of you if you are a student. There are two widely used classifications of limestone rocks that have proven to be most practical, those of Dunham (1962) and Folk (1962).

Both

"Rely on the concept of textural maturity, where the texture is believed to reflect the energy level in the depositional environment"
(Wright, 1992).

I am doing field work and mapping in carbonates (mainly limestones, few marls) meaning I need an easily applicable method to name my sample in the field without knowing much about its history. The original Dunham classification is based on depositional texture and can easily be applied in the field without major equipment (magnifying glass, acid, a scale). Folk's classification may be more detailled but generally, with exceptions, requires a lab with thin sections or peels. For the above reasons I will restrict myself to introducing you to the expanded and revised Dunham classification of Wright (1992).

Dunham originally included five textural classes: mudstone, wackestone, packstone, grainstone and boundstone. A sixth class was included for rocks without recognisable depositional texture and termed crystalline carbonate.

In 1971 the Dunham classification was expanded by Embry and Klovan to include the size aspects of grains larger than 2 mm and the organical boundstone was extended to include frame-, bind- and bafflestone.

Wright revised this classification in 1992 and included diagenetic changes of depositional texture to further appreciate the impact of diagenesis on limestones. The terms mud-, float- and rudstone were revised and five diagenetic classes added: cementstone, condensed and fitted grainstone, sparstone and mircosparstone. In my own experience the diagenetic classes might require lab work though because they are not always easily recognisable in the field.

Any good textbook discussion carbonate rocks probably includes the following figure. The essential description and characteristics are shown in the figure and I recommend for anyone interested to either consult the original articles or any good textbook on carbonates for more detailled information than I will give in this post.


The revised classification of limestones (Wright, 1992)

You should keep in mind that textures in limestones are the result from the interplay of three major factors: depositional regime, biological activity and diagenesis.


A fundamental step in classification is seperating grain-support from matrix-support which is not always as easy as it may sound and depends on the two-dimensional aspect of how we look at a rock surface. As mentioned above the term mudstone was revised by Wright to calcimudstone to emphasise the difference from siliciclastic sediments and avoid confusion. Dispite similiar grain-size calcimudstone and Folk's micrite are not synonymous and should not be confused! Also you should not think that calcimudstone or wackestones are analogous to their siliciclastic counterparts. Hydraulic processes would not lead to such mixtures of fine and coarse grained materials (usually) as we can observe in wackestones. It might be interesting to remember that at least some packstones are likely compacted and dewatered original wackestones. Grainstones are matrix-free and grain-supported. Mud has either been removed or never formed, usually indicating high energy environments. However, they can also form by diagenetic processes, i.e. desiccation on exposure. Float- and Rudstones are basicly the coarse-grained counterparts of wackestone and pack- to grainstones. Due to almost synonymous usage and semantical problems Wright revised the original bindstones of Embry and Klovan and named them boundstone, like in the original Dunham classification that included all organical limestones into this class though. Beware not to confuse one with another! Also I am sure a few of you were wondering what happened to the bindstones on the above figures. The remaining biogenic textural classes did not change, however, Wright noted how the term bafflestone might be obsolete because it is virtually impossible to show that organisms baffled sediment.

Cementstone is almost totally composed of fibrous cement that is commonly replaced or recrystallised. Grains or in-situ biogenic materials do not form a framework.

Grain-supported textures are often changed in burial which produces partially sutured grains (condensed grainstones) or transforms all grain contacts to microstylolites (fitted grainstones).

Limestones that have lost their depositional texture due to recrystallisation or replacement are divided into sparstones and microsparstones, depending on their crystal size being larger or smaller than 10 microns. I still use crystalline limestone because they are rare and hard to tell apart where I am working.

To conclude remember this quote:

"There are too many papers that simply conclude that the carbonate rocks of a given study can be arranged according to the classification of one or more authors. All too often these workers have failed to recognize that a classification is a simple tool for organizing information, not the source of a conclusion" (Blatt et al. 1972).

Bold passages emphasised by me.

A very good read on carbonates and carbonate classification can be found here and on the USC Sequence Stratigraphy Web.


Important references used:
  • Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. Mem. Am. Assoc. Pet. Geol., 1: 108-121.
  • Flügel, E., 2004. Microfacies of Carbonate Rocks. Springer Verlag, Berlin Heidelberg, 976 pp.
  • Folk, R.L., 1959. Practical petrographic classification of limestones. Bull. Am. Assoc. Pet. Geol., 43: 1-38.
  • Wright, V.P., 1992. A revised classification of limestones. Sediment. Geol., 76: 177-185 doi:10.1016/0037-0738(92)90082-3

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