Wackestone

{{short description|Mud-supported carbonate rock that contains greater than 10% grains}}

Under the Dunham classification (Dunham, 1962Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. In: W.E. Ham (Ed.), Classification of Carbonate Rocks. American Association of Petroleum Geologists Memoir. American Association of Petroleum Geologists, Tulsa, Oklahoma, pp. 108–121.) system of limestones, a wackestone is defined as a mud-supported carbonate rock that contains greater than 10% grains. Most recently, this definition has been clarified as a carbonate-dominated rock in which the carbonate mud (<63 μm) component supports a fabric comprising 10% or more very fine-sand grade (63 μm) or larger grains but where less than 10% of the rock is formed of grains larger than sand grade (>2 mm).{{Cite journal|last=Lokier|first=Stephen W.|last2=Al Junaibi|first2=Mariam|date=2016-12-01|title=The petrographic description of carbonate facies: are we all speaking the same language?|journal=Sedimentology|language=en|volume=63|issue=7|pages=1843–1885|doi=10.1111/sed.12293|issn=1365-3091|doi-access=free}}

The identification of wackestone

[[File:Wackestone.svg|thumb|350px|Schematic wackestone as seen in thin section under the petrographic microscope.

Type of carbonaceous rock according to the depositional texture:

Alloctonous carbonates – Original components not bound at the deposition time.

Less than 10% of components larger than sand size (> 2 mm)

Contains carbonate mud (micrite, silt/clay size <63 μm)

Fabric supported by carbonate mud (micrite, <63 μm)

10% or more composed of 63 μm or greater grains

Legend:

Dotted background: micritic matrix.

Curved blue particles: bioclasts (indeterminate fossils, e.g. bivalve fragments).

Blue cones: bioclasts (e.g. fossils of gastropods)

Black spheroids: bioclasts (pellets). ]]

A study of the adoption and use of carbonate classification systems by Lokier and Al Junaibi (2016) highlighted that the most common problem encountered when describing a wackestone is to incorrectly estimate the volume of 'grains' in the sample – in consequence, misidentifying wackestone as mudstone or vice versa. The original Dunham classification (1962) defined the matrix as clay and fine-silt size sediment <20 μm in diameter. This definition was redefined by Embry & Klovan (1971)Embry, A.F. and Klovan, J.E., 1971. A Late Devonian reef tract on Northeastern Banks Island, NWT. Bulletin of Canadian Petroleum Geology, 19(4)), 730–781. to a grain size of less than or equal to 30 μm. Wright (1992)Wright, V.P., 1992. A revised classification of limestones. Sedimentary Geology, 76(3–4), 177–185. proposed a further increase to the upper limit for the matrix size in order to bring it into line with the upper limit for silt (62 μm).

References

External links

http://strata.geol.sc.edu/thinsections/Carbonate-glossary.html

Category:Limestone