2010年3月8日

Introduction to limestone classifications

 Introduction to limestone classifications

Although many classifications have been proposed for carbonate rocks and sediments, only two — the Folk (1959/62) and Dunham (1962) classifications — have successfully met the test of time (along with two others that are variants of the Dunham scheme). All four schemes are based on the distinction of three fundamental components: grains (skeletal fragments, ooids, pellets/peloids, intraclasts, and non-carbonate detritus), matrix or carbonate mud, and open pores or sparry-calcite-fi lled primary interparticle porosity (see diagram on previous page). The differences between the classifications are mainly that Folk uses the relative percentages of grains and matrix, Dunham as well as Embry and Klovan use mud- versus grain-supported fabrics, and Wright uses a more genetic division into biological, diagenetic, and depositional fabrics.

Folk (1959/62) classification        

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).

Dunham (1962) classification

In contrast, Dunham's classification (figures above) and its modification by Embry and Klovan (1971) and James (1984) deals with depositional texture. For this reason, his scheme may be better suited for rock descriptions that employ a hand lens or binocular microscope. For example, if the grains of a limestone are touching one another and the sediment contains no mud, then the sediment is called a grainstone. If the carbonate is grain supported but contains a small percentage of mud, then it is known as a packstone. If the sediment is mud supported but contains more than 10 percent grains, then it is known as a wackestone, and if it contains less than 10 percent grains and is mud supported, it is known as a mudstone.

Embry & Klovan (1971) classification

modified the Dunham scheme by further subdividing coarse-grained skeletal deposits and organically formed or organically bound carbonate rocks. The five new terms add to the descriptive capability of the Dunham classification in the area of biogenic deposits, especially reefs and bioherms.

Wright (1992) classification

This summary diagram, showing the essential components of the Wright classification, emphasizes the distinction between carbonate strata influenced by depositional processes (physical), biological processes, or diagenetic processes (both synsedimentary and postdepositional).
Although the names are mainly derived from the Dunham or Embry-Klovan classifi cations, several useful new terms were coined, including “cementstone”, “condensed grainstone”, and “fitted grainstone” for cement-rich or chemically compacted limestones.


References
Dunham, R. J., 1962, Classification of carbonate rocks according to depositional texture. In: Ham, W. E. (ed.), Classification of carbonate rocks: American Association of Petroleum Geologists Memoir, p. 108-121.
Embry, AF, and Klovan, JE, 1971, A Late Devonian reef tract on Northeastern Banks Island, NWT: Canadian Petroleum Geology Bulletin, v. 19, p. 730-781.
Folk, R.L., 1959, Practical petrographic classification of limestones: American Association of Petroleum Geologists Bulletin, v. 43, p. 1-38.
Folk, R.L., 1962, Spectral subdivision of limestone types, in Ham, W.E., ed., Classification of Carbonate Rocks-A Symposium: American Association of Petroleum Geologists Memoir 1, p. 62-84.
James, N.P., 1984, Shallowing-upward sequences in carbonates, in Walker, R.G., ed., Facies Models: Geological Association of Canada, Geoscience Canada, Reprint Series 1, p. 213–228.
Scholle, P. A. and Ulmer-Scholle, D. S, 2003, A Color Guide to the Petrography of Carbonate Rocks: AAPG Memoir 77, 474 p