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)[1] 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.[2]

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:
    • Shale — exhibits lamination or fissility.
    • Argillite — has undergone low-grade metamorphism.[2]
East Beach 1 2006
Mudstone on east beach of Lyme Regis, England

Carbonate mudstone

A Mudstone: few small components in a micritic matrix, width of picture is 32 mm

In the Dunham classification (Dunham, 1962[3]) 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.[4]

The identification of carbonate mudstone

14 x2 PPL
Thin section photomicrograph of carbonate mudstone

A recent study by Lokier and Al Junaibi (2016)[4] 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)[3] defined the matrix as clay and fine-silt size sediment <20 μm in diameter. This definition was redefined by Embry & Klovan (1971[5]) to a grain size of less than or equal to 30 μm. Wright (1992[6]) 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)[7]
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)

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.[8] Also reported, the very soluble element boron was detected for the first time on Mars.[8] In June 2018, NASA reported that Curiosity had detected kerogen and other complex organic compounds from mudstone rocks approximately 3.5 billion years old.[9][10][11][12][13][14][15][16]

See also


  1. ^ Verruijt, Arnold (2018). An Introduction to Soil Mechanics, Theory and Applications of Transport in Porous Media. Springer. pp. 13–14. ISBN 978-3-319-61185-3.
  2. ^ a b Blatt, H., and R.J. Tracy, 1996, Petrology. New York, New York, W. H. Freeman, 2nd ed, 529 pp. ISBN 0-7167-2438-3
  3. ^ a b 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.
  4. ^ a b 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.
  5. ^ Embry, Ashton F.; Klovan, J. Edward (1971-12-01). "A late Devonian reef tract on northeastern Banks Island, N.W.T". Bulletin of Canadian Petroleum Geology. 19 (4): 730–781. ISSN 0007-4802.
  6. ^ Wright, V. P. (1992-03-01). "A revised classification of limestones". Sedimentary Geology. 76 (3): 177–185. doi:10.1016/0037-0738(92)90082-3.
  7. ^ Staff (December 13, 2016). "PIA21146: Mudstone Mineralogy from Curiosity's CheMin, 2013 to 2016". NASA. Retrieved December 16, 2016.
  8. ^ a b Cantillo, Laurie; Brown, Dwayne; Webster, Guy; Agle, DC; Tabor, Abigail; Mullane, Laura (December 13, 2016). "Mars Rock-Ingredient Stew Seen as Plus for Habitability". NASA. Retrieved December 14, 2016.
  9. ^ Brown, Dwayne; Wendel, JoAnna; Steigerwald, Bill; Jones, Nancy; Good, Andrew (June 7, 2018). "Release 18-050 - NASA Finds Ancient Organic Material, Mysterious Methane on Mars". NASA. Retrieved June 11, 2018.
  10. ^ NASA (June 7, 2018). "Ancient Organics Discovered on Mars - video (03:17)". NASA. Retrieved June 11, 2018.
  11. ^ Wall, Mike (June 7, 2018). "Curiosity Rover Finds Ancient 'Building Blocks for Life' on Mars". Retrieved June 11, 2018.
  12. ^ Chang, Kenneth (June 7, 2018). "Life on Mars? Rover's Latest Discovery Puts It 'On the Table' - The identification of organic molecules in rocks on the red planet does not necessarily point to life there, past or present, but does indicate that some of the building blocks were present". The New York Times. Retrieved June 11, 2018.
  13. ^ Voosen, Paul (June 7, 2018). "NASA rover hits organic pay dirt on Mars". Science. Retrieved June 11, 2018.
  14. ^ ten Kate, Inge Loes (June 8, 2018). "Organic molecules on Mars". Science. 360 (6393): 1068–1069. doi:10.1126/science.aat2662. PMID 29880670.
  15. ^ Webster, Christopher R.; et al. (June 8, 2018). "Background levels of methane in Mars' atmosphere show strong seasonal variations". Science. 360 (6393): 1093–1096. doi:10.1126/science.aaaq0131 (inactive 2019-03-07). Retrieved June 11, 2018.
  16. ^ Eigenbrode, Jennifer L.; et al. (June 8, 2018). "Organic matter preserved in 3-billion-year-old mudstones at Gale crater, Mars". Science. 360 (6393): 1096–1101. doi:10.1126/science.aaas9185 (inactive 2019-03-07). Retrieved June 11, 2018.

"Argillite" may also refer to Argillite, Kentucky.

Argillite ( ) is a fine-grained sedimentary rock composed predominantly of indurated clay particles. Argillaceous rocks are basically lithified muds and oozes. They contain variable amounts of silt-sized particles. The argillites grade into shale when the fissile layering typical of shale is developed. Another name for poorly lithified argillites is mudstone. These rocks, although variable in composition, are typically high in aluminium and silica with variable alkali and alkaline earth cations. The term pelitic or pelite is often applied to these sediments and rocks. Metamorphism of argillites produces slate, phyllite, and pelitic schist.

Caleta Herradura Formation

Caleta Herradura Formation (Spanish: Formación Caleta Herradura) is a geologic formation of Late Miocene (Montehermosan) age, cropping out on the Mejillones Peninsula in northern Chile. The erosion at the Coastal Cliff of northern Chile have created particularly good exposures of Caleta Herradura Formation. The formation deposited in a half graben within Mejillones Peninsula. The formation rests nonconformably on the Jorgino Formation.

Cheuquemó Formation

Cheuquemó Formation (Spanish: Formación Cheuquemó) is a geological formation of sedimentary rock in south-central Chile. The sediments of the formation were deposited during the Late Oligocene and Early Miocene epochs. The formations lower sections are made up of conglomerate, then successions of sandstone, tuff and mudstone rich in organic material follows. The formation indicates that sedimentation occurred in a estuarine (paralic) and other non-marine (continental) environments. It contains fossils of the following genera: Mytilus, Cardium and Turritella. Stratigraphically it overlies the Bahía Mansa Metamorphic Complex and underlies the Miocene Santo Domingo Formation.

Estratos de Pupunahue

Estratos de Pupunahue is the name given to the sedimentary strata of Oligocene-Miocene age that crop out in Pupunahue and Mulpún near Valdivia, Chile. Outside this locality Estratos de Pupunahue extends below the surface over a larger area. The thickness of the strata varies from a few meters to 530 meters. The strata were initially described by Henning Illies.

The strata are made up of conglomerate, sandstone and mudstone (Chilean Spanish: fangolita). The clast of the conglomerates are made up of metamorphic rock and the disposition of the conglomerates varies from clast-supported to matrix-supported. The sandstone and mudstone contain layers of lignite coal that exceed 30 cm in thickness.Coal layers found in the Estratos de Pupunahue have been exploited in the mines of Catamutún, Pupunahue and Mulpún ("Mulpun Beds").The strata are very similar to the Cheuquemó Formation found further north, with the sole difference that the fossil assemblage in both seems to indicate different ages. While Cheuquemó Formation is possibly about 14 million years old (Miocene), Estratos de Pupunahue are 35–25 million years old.

Estratos de San Pedro

Estratos de San Pedro is the name given to the sedimentary strata of Paleogene age that crop out along San Pedro River, southern Chile. The strata were initially described by Juan Brüggen and later briefly investigated by Henning Illies who estimated their thickness at 1,000 metres (3,300 ft).

The strata are made up of conglomerate, sandstone and mudstone (Chilean Spanish: fangolita). The clasts of the conglomerates are made up of metamorphic rock and the disposition of the conglomerates varies from clast-supported to matrix-supported. The sandstone and mudstone contains layers of lignite coal that exceed 30 centimetres (12 in) in thickness.

Lebu Group

Lebu Group is stratigraphic unit of Arauco Basin in south-central Chile. The group consists of a sequence of four formations, of both marine and nonmarine origin, deposited between the Early Paleocene and Middle Eocene.

Lukachukai Mountains

The Lukachukai Mountains are a mountain range in northeast Arizona, entirely located on the Navajo Nation. The highest point of the range is an unnamed point at 9466 feet (2885 meters) above sea level. While open during the winter, no road maintenance is performed and chains or four wheel drive with good snow tires are essential due to grades of up to 14% in many places.

The Lukachukai Mountain trail gives access to some of the scenic red-rock high country here. The trailhead is located just south of Buffalo Pass on Indian Route 13, a paved highway. The trail is passable to OHVs in dry weather. A permit is required from the Navajo Nation government.There is a belt of uranium deposits that crosses the Lukachukai Mountains. The ore belt is within a bed of lenticular sandstone and mudstone that apparently provided sufficient permeable sandstone for the passage of ore solutions, and sufficient impermeable mudstone to restrict movement of solutions to particular sandstone beds. An unusual concentration of organic material (carbonized wood?) served as a precipitation agent.


Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and silt. The dominant carbonate mineral in most marls is calcite, but other carbonate minerals such as aragonite, dolomite, and siderite may be present. Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay and calcium carbonate, formed under freshwater conditions; specifically an earthy substance containing 35–65% clay and 65–35% carbonate. It also describes a habit of coralline red alga. The term is today often used to describe indurated marine deposits and lacustrine (lake) sediments which more accurately should be named 'marlstone'. Marlstone is an indurated (resists crumbling or powdering) rock of about the same composition as marl, more correctly called an earthy or impure argillaceous limestone. It has a blocky subconchoidal fracture, and is less fissile than shale. The term 'marl' is widely used in English-language geology, while the terms Mergel and Seekreide (German for "lake chalk") are used in European references.

The lower stratigraphic units of the chalk cliffs of Dover consist of a sequence of glauconitic marls followed by rhythmically banded limestone and marl layers. Upper Cretaceous cyclic sequences in Germany and marl–opal-rich Tortonian-Messinian strata in the Sorbas basin related to multiple sea drawdown have been correlated with Milankovitch orbital forcing.Marl as lacustrine sediment is common in post-glacial lake-bed sediments, often found underlying peat bogs. It has been used as a soil conditioner and acid soil neutralizing agent.

Marros Group

The Marros Group is the name given to a suite of rocks of Namurian age laid down during the Carboniferous Period in South Wales. These rocks were formerly known as the Millstone Grit Series but are now distinguished from the similar but geographically separate rock sequences of the Pennines and Peak District of northern England and northeast Wales by this new name.

The Group comprises a thick unit of coarse sandstone known as the Twrch Sandstone (formerly the ‘Basal Grit’) which is overlain by the Bishopston Mudstone and the Telpyn Point Sandstone. The mudstones of these latter two formations was formerly known as the ‘Middle Shales’, a name reflecting the position of this sequence sandwiched between the Basal Grit below and the Farewell Rock, the lowermost sandstone of the South Wales Coal Measures, above. The mudstone itself contains a few bands of sandstone such as the ‘Twelve Foot Sandstone’ and locally the ‘Cumbriense Sandstone’.

Mercia Mudstone Group

Lower & Middle Keuper Marl, Keuper Waterstones and Upper & Lower Keuper Saliferous Beds redirect here.The Mercia Mudstone Group is an early Triassic lithostratigraphic group (a sequence of rock strata) which is widespread in Britain, especially in the English Midlands – the name is derived from the ancient kingdom of Mercia which corresponds to that area. It is frequently encountered in older literature as the Keuper Marl or Keuper Marl Series.The Mercia Mudstone Group is now divided into five formations recognised and mappable across its entire outcrop and subcrop. The formations are a mix of mudstones, siltstones, sandstones and halites. Historically this sequence of rocks has been subdivided in different ways with different names in each of the basinal areas in which it is found. Increasing knowledge of the sequences and the more recent development of seamless electronic mapping by the British Geological Survey (BGS) necessitated a reappraisal of these divisions. A report published by BGS in 2008 recommended the abandonment of previous divisions and naming schemes in favour of a simpler approach which, having now been adopted, is set out below.

Blue Anchor Formation

Branscombe Mudstone Formation

Arden Sandstone Formation

Sidmouth Mudstone Formation

Tarporley Siltstone FormationClearly, older schemes will remain in maps and literature well into the future, providing a source of potential confusion. An example might be the Arden Sandstone Formation which previously enjoyed lower status as a member and also higher status as a group.

Morrison Formation

The Morrison Formation is a distinctive sequence of Upper Jurassic sedimentary rock found in the western United States which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandstone, siltstone, and limestone and is light gray, greenish gray, or red. Most of the fossils occur in the green siltstone beds and lower sandstones, relics of the rivers and floodplains of the Jurassic period.

It is centered in Wyoming and Colorado, with outcrops in Montana, North Dakota, South Dakota, Nebraska, Kansas, the panhandles of Oklahoma and Texas, New Mexico, Arizona, Utah, and Idaho. Equivalent rocks under different names are found in Canada. It covers an area of 1.5 million square kilometers (600,000 square miles), although only a tiny fraction is exposed and accessible to geologists and paleontologists. Over 75% is still buried under the prairie to the east, and much of its western paleogeographic extent was eroded during exhumation of the Rocky Mountains.

It was named after Morrison, Colorado, where the first fossils in the formation were discovered by Arthur Lakes in 1877. That same year, it became the center of the Bone Wars, a fossil-collecting rivalry between early paleontologists Othniel Charles Marsh and Edward Drinker Cope. In Colorado, New Mexico, and Utah, the Morrison Formation was a major source of uranium ore.

Mud cracks on Mars

In January 2017, scientists announced the possible discovery of mud cracks in Gale Crater on Mars. The Curiosity Rover imaged what may be the first mud cracks (desiccation cracks) ever found on Mars. They may have been formed from drying mud. The site, called “Old Soaker,” was within an exposure of Murray formation mudstone on lower Mount Sharp.

It is hypothesized that these cracks formed more than 3 billion years ago and then were buried by more sediment. All this material eventually turned into rock. Later wind erosion removed the layers that covered the cracked layer. The cracks were filled with material which was resistant to later erosion. This erosion resistant material formed raised ridges, as some of the surrounding layer was removed.

This is the first sighting of mud cracks. Previously, Curiosity has examined cracks and ridges of different shapes that were made by groundwater carrying minerals, such as calcium sulfate. Cracks for this process were caused by the pressure of overlying sediments fracturing rock.

Gale Crater held ancient lakes that varied in depth and area over time, and sometimes disappeared. Mud cracks show that there were dry times when lakes disappeared. Besides this evidence of mud, Curiosity has found evidence of ancient lakes in older layers and also in younger mudstone. Nathan Stein, a graduate student at the California Institute of Technology led the investigation.


Mudrocks are a class of fine grained siliciclastic sedimentary rocks. The varying types of mudrocks include: siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than 0.0625 mm (1/16th mm or 0.0025 inches) and are too small to study readily in the field. At first sight the rock types look quite similar; however, there are important differences in composition and nomenclature. There has been a great deal of disagreement involving the classification of mudrocks. There are a few important hurdles to classification, including:

Mudrocks are the least understood, and one of the most understudied sedimentary rocks to date

It is difficult to study mudrock constituents, due to their diminutive size and susceptibility to weathering on outcrops

And most importantly, there is more than one classification scheme accepted by scientistsMudrocks make up fifty percent of the sedimentary rocks in the geologic record, and are easily the most widespread deposits on Earth. Fine sediment is the most abundant product of erosion, and these sediments contribute to the overall omnipresence of mudrocks. With increased pressure over time the platey clay minerals may become aligned, with the appearance of parallel layering (fissility). 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 either to the original texture or to the disruption of layering by burrowing organisms in the sediment prior to lithification.

From the beginning of civilization, when pottery and mudbricks were made by hand, to now, mudrocks have been important. The first book on mudrocks, Geologie des Argils by Millot, was not published until 1964; however, scientists, engineers, and oil producers have understood the significance of mudrocks since the discovery of the Burgess Shale and the relatedness of mudrocks and oil. Literature on this omnipresent rock-type has been increasing in recent years, and technology continues to allow for better analysis.

Old Red Sandstone

The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the northeastern seaboard of North America. It also extends northwards into Greenland and Svalbard. In Britain it is a lithostratigraphic unit (a sequence of rock strata) to which stratigraphers accord supergroup status and which is of considerable importance to early palaeontology. For convenience the short version of the term, ORS is often used in literature on the subject. The term was coined to distinguish the sequence from the younger New Red Sandstone which also occurs widely throughout Britain.

Pan de Azúcar Formation

Pan de Azúcar Formation (Spanish: Formación Pan de Azúcar, Sugar-loaf formation) is a geologic formation of Hettangian–Sinemurian (Jurassic) age made up of chiefly by sandstone, tuff, mudstone and limestone. The formation is located in the Coast Range of northern Chile. The formation interdigitates and is coeval with the Posada de los Hidalgo Formation. It concordantly overlies the Cifuncho Formation and is overlain by the La Negra Formation.At least one location the formation is intruded by a roughly coeval dacite dyke which adds to the evidence that the formation was coeval with the early stages of "Andean" volcanism.

Parga Formation

Parga Formation (Spanish: Formación Parga) is a geological formation of sedimentary rock in south-central Chile. The sediments of the formation were deposited during the Late Oligocene and Middle Miocene epochs. The formation's lower sections are made up of conglomerate, sandstone and mudstone some of which is rich in organic material. Additionally there are thin beds of tuff and coal. The formation's composition indicates that sedimentation occurred in a estuarine (paralic) and marine environments. Stratigraphically it overlies the Bahía Mansa Metamorphic Complex and is similar in age and type to Lacui Formation to the south and Cheuquemó and Santo Domingo Formation to the north. It is overlain across an angular unconformity by Pliocene or Quaternary sediments. The formation is intruded by porphyritic trachyte of Oligocene to Miocene age (Ancud Volcanic Complex). The outcrops of the formation are restricted to a NW-SE strip near Caleta Parga north of the estuary of Maullín River.

Santo Domingo Formation

Santo Domingo Formation (Spanish: Formación Santo Domingo) is a mainly marine Miocene sedimentary formation located in south–central Chile. The formation was defined by R. Martínez Pardo and Mario Pino in 1979 and named after the roadcut locality they studied about 19 kilometres (12 mi) southeast of Valdivia. Sediments of the formation accumulated in Valdivia and Osorno–Llanquihue Basin.The formations overlies basement consisting of metamorphic and igneous rocks; the Bahía Mansa Metamorphic Complex and Cretaceous granitoids respectively. In parts, it further overlies the coal–bearing Pupunahue–Catamutún Formation. The sedimentary facies of the Santo Domingo Formation are composed of sandstone, siltstone and mudstone and smaller amounts of conglomerate. The formation underlies Pliocene and Quaternary sediments.

Williams Fork Formation

The Williams Fork Formation is a Campanian (Edmontonian) geologic formation of the Mesaverde Group in Colorado. Dinosaur remains are among the fossils, most notably Pentaceratops sternbergii, that have been recovered from the formation, although none have yet been referred to a specific genus. Other fossils found in the formation are ammonites, Neosuchia, and the mammal Meniscoessus collomensis.


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