Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.063 millimetres (0.0025 in) with individual grains too small to be distinguished without a microscope. With increased pressure over time, the platy clay minerals may become aligned, with the appearance of fissility or parallel layering. This finely bedded material that splits readily into thin layers is called shale, as distinct from mudstone. 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 to lithification. Mud rocks such as mudstone and shale account for some 65% of all sedimentary rocks. 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.
Mudstone can be separated into these categories:
Siltstone — more than half of the composition is silt-sized particles.
Claystone — more than half of the composition is clay-sized particles.
Mudstone — hardened mud; a mix of silt and clay sized particles. Mudstone can include:
A Mudstone: few small components in a micritic matrix, width of picture is 32 mm
In the Dunham classification (Dunham, 1962) system of limestones, a mudstone is defined as a mud-supported carbonate rock that contains less than 10% grains. Most recently, this definition has been clarified as a matrix-supported carbonate-dominated rock composed of more than 90% carbonate mud (<63 μm) component.
The identification of carbonate mudstone
Thin section photomicrograph of carbonate mudstone
A recent study by Lokier and Al Junaibi (2016) 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 as wackestone and 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) to a grain size of less than or equal to 30 μm. Wright (1992) 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).
Mudstone mineralogy on Mars
Curiosity rover - mudstone mineralogy - 2013 to 2016 on Mars (CheMin; December 13, 2016) 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, see image)
Tonstein – A hard, compact sedimentary rock that is composed mainly of kaolinite or, less commonly, other clay minerals
^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.
^ abBlatt, H., and R.J. Tracy, 1996, Petrology. New York, New York, W. H. Freeman, 2nd ed, 529 pp. ISBN0-7167-2438-3
^ 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.