Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were or . Mudstone is distinguished from shale by its lack of fissility.[Blatt, H., and R.J. Tracy, 1996, Petrology. New York, New York, W. H. Freeman, 2nd ed, 529 pp. ]
The term mudstone is also used to describe carbonate rocks (limestone or dolomite) that are composed predominantly of carbonate mud.
The NASA Curiosity rover has found deposits of mudstone on Mars that contain organic substances such as propane, benzene and toluene.
Definition
There is not a single definition of mudstone that has gained general acceptance,
[Boggs 2006, p.143] though there is wide agreement that mudstones are fine-grained sedimentary rocks, composed mostly of silicate grains with a
Particle size less than .
Individual grains this size are too small to be distinguished without a microscope, which means that most classifications emphasize texture rather than mineral composition,
[Boggs 2006, p.143] and mudstones have historically received less attention from petrologists than have
.
The simplest definition is that a mudstone is a fine-grained clastic sedimentary rock that is not laminated or fissile.
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.
Another definition is that mudstone is a sedimentary rock in which neither silt, clay, nor coarser grains is predominant.
Rock of this composition that does show laminations or fissility is sometimes described as
mudshale rather than
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. 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.
When the mineral composition of mudstones has been determined, using such techniques as scanning electron microscopy, electron probe microanalysis, or X-ray diffraction analysis, they have been found to be composed primarily of , quartz, and , with a variety of accessory minerals.[Boggs 2006, pp.140-143]
Carbonate mudstone
In the Dunham classification (Dunham, 1962
[Dunham, 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
, 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
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
On December 13, 2016,
NASA reported further evidence supporting habitability on the planet
Mars as the
Curiosity rover climbed higher, studying younger layers, on
Mount Sharp.
Also reported, the very soluble element
boron was detected for the first time on Mars.
In June 2018, NASA reported that
Curiosity had detected
kerogen and other complex
from mudstone rocks approximately 3.5 billion years old.
See also
-
Mudstone on planet Mars
-
Aeolis quadrangle
-
Composition of Mars
-
Timeline of Mars Science Laboratory
