Grainstone

Under the Dunham classification (Dunham, 1962)[1] system of limestones, a grainstone is defined as a grain-supported carbonate rock that contains less than 1% mud-grade material. This definition has recently been clarified as a carbonate-dominated rock that does not contain any carbonate mud and where less than 10% of the components are larger than 2 mm. The spaces between grains may be empty (pores) or filled by cement.

Thin section photomicrograph of calcite cemented coarse-grained ooid grainstone
Grainstone in the Dunham Classification (Brassfield Formation near Fairborn, Ohio). Grains are crinoid fragments.

The identification of grainstone

The presence of any primary carbonate mud precludes a classification of grainstone. A study of the use of carbonate classification systems by Lokier and Al Junaibi[2] highlighted that the most common source of confusion in the classification of grainstone was to misidentify fine-grained internal micrite, generated by in-situ processes, as clay–silt grade sediment - thus resulting in the misidentification of grainstone as packstone. Failure to correctly determine the size and abundance of component grains greater than two millimeters was also a source of error.

Dunham's original definition of grainstone stated that it must contain less than 1% mud to fine-silt grade (<20 μm) sediment. Embry and Klovan[3] and Wright [4] reduced the permitted amount of carbonate mud in a grainstone to zero. Given that grainstone facies are interpreted to have been deposited under high-energy conditions, it is sensible to preclude the presence of primary carbonate mud from this classification.

Thin section photomicrograph of calcite cemented ooid grainstone

References
  1. Dunham, R.J. (1962) Classification of carbonate rocks according to depositional texture. In: Classification of Carbonate Rocks (Ed. W.E. Ham), Am. Assoc. Pet. Geol. Mem., 1, 108–121.
  2. Lokier, Stephen W.; Al Junaibi, Mariam (2016-12-01). "The petrographic description of carbonate facies: are we all speaking the same language?". Sedimentology. 63 (7): 1843–1885. doi:10.1111/sed.12293. ISSN 1365-3091.
  3. Embry, A.F. and Klovan, J.E. (1971) A Late Devonian reef tract on Northeastern Banks Island, NWT. Bull. Can. Pet. Geol., 19, 730–781.
  4. Wright, V.P. (1992) A revised classification of limestones. Sed. Geol., 76, 177–185.
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