The termmudstone is also used to describecarbonate rocks (limestone ordolomite) that are composed predominantly of carbonate mud.[3] However, in most contexts, the term refers tosiliciclastic mudstone, composed mostly of silicate minerals.[2]
There is not a single definition of mudstone that has gained general acceptance,[5] though there is wide agreement that mudstones are fine-grained sedimentary rocks, composed mostly of silicate grains with agrain size less than 0.063 millimetres (0.0025 in).[6] Individual grains this size are too small to be distinguished without a microscope, which means that most classifications emphasize texture rather than mineral composition,[5] and mudstones have historically received less attention from petrologists than havesandstones.[7] The simplest definition is that a mudstone is a fine-grainedclastic sedimentary rock that is notlaminated orfissile.[5] Most definitions also include a requirement that the rock contain significant amounts of both silt- and clay-sized grains. One common requirement is that a mudstone is a mudrock (a rock containing more than 50% silt- to clay-sized particles) in which between a third and two-thirds of the mud (silt and clay) fraction is clay particles.[7][8] Another definition is that mudstone is a sedimentary rock in which neither silt, clay, nor coarser grains is predominant.[9] Rock of this composition that does show laminations or fissility is sometimes described asmudshale rather thanmudstone.[8]
The lack of fissility or layering in mudstone may be due to either original texture or the disruption of layering by burrowing organisms in the sediment prior tolithification. Mudstone looks like hardened clay and, depending upon the circumstances under which it was formed, it may show cracks or fissures, like a sun-baked clay deposit.[1]
A mudstone: few small components in a micritic matrix, width of picture is 32 mm
In theDunham classification (Dunham, 1962[11]) system oflimestones, amudstone is defined as a mud-supportedcarbonate rock that contains less than 10% grains. Most recently, this definition has been clarified asa matrix-supported carbonate-dominated rock composed of more than 90% carbonate mud (<63 μm) component.[3]
Thin section photomicrograph of carbonate mudstone
A recent study by Lokier and Al Junaibi (2016)[3] has highlighted that the most common problems encountered when describing a mudstone is to incorrectly estimate the volume of 'grains' in the sample - in consequence, misidentifying mudstone aswackestone and vice versa. The original Dunham classification (1962)[11] defined the matrix as clay and fine-silt size sediment <20 μm in diameter. This definition was redefined by Embry & Klovan (1971[12]) to a grain size of less than or equal to 30 μm. Wright (1992[13]) 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 (63 μm).
Curiosity rover - mudstone mineralogy - 2013 to 2016 onMars (CheMin; December 13, 2016)[14] NOTE: JK for "John Klein", CB for "Cumberland". CH for "Confidence Hills", MJ for "Mojave", TP for "Telegraph Peak", BK for "Buckskin", OD for "Oudam", MB for "Marimba", QL for "Quela", and SB for Sebina. (For locations/drillings, seeimage)
^Verruijt, Arnold (2018).An Introduction to Soil Mechanics, Theory and Applications of Transport in Porous Media. Springer. pp. 13–14.ISBN978-3-319-61185-3.
^abPotter, Paul Edwin; Maynard, James; Pryor, Wayne A. (1980).Sedimentology of shale : study guide and reference source. New York: Springer-Verlag. p. 14.ISBN0387904301.
^abDunham, 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.
