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Sedimentary rock
Chattanooga Shale (Upper Devonian; Burkesville West Rt. 90 roadcut, Kentucky, USA) 25 (40541681100).jpg
Clay minerals and quartz

Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al 2 Si 2 O 5(OH)4) and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. [1] Shale is characterized by its tendency to split into thin layers (laminae) less than one centimeter in thickness. This property is called fissility . [1] Shale is the most common sedimentary rock. [2]


The term shale is sometimes applied more broadly, as essentially a synonym for mudrock, rather than in the narrower sense of clay-rich fissile mudrock. [3]


Shale typically exhibits varying degrees of fissility. Because of the parallel orientation of clay mineral flakes in shale, it breaks into thin layers, often splintery and usually parallel to the otherwise indistinguishable bedding planes. [4] Non-fissile rocks of similar composition and particle size (less than 0.0625 mm) are described as mudstones (1/3 to 2/3 silt particles) or claystones (less than 1/3 silt). Rocks with similar particle sizes but with less clay (greater than 2/3 silt) and therefore grittier are siltstones. [4] [5]

Sample of drill cuttings of shale while drilling an oil well in Louisiana, United States. Sand grain = 2 mm in diameter Drill cuttings - Annotated - 2004.jpg
Sample of drill cuttings of shale while drilling an oil well in Louisiana, United States. Sand grain = 2 mm in diameter

Composition and color

Color chart for shale based on oxidation state and organic carbon content Mudrock Colors.jpg
Color chart for shale based on oxidation state and organic carbon content

Shales are typically gray in color and are composed of clay minerals and quartz grains. The addition of variable amounts of minor constituents alters the color of the rock. Red, brown and green colors are indicative of ferric oxide (hematite – reds), iron hydroxide (goethite – browns and limonite – yellow), or micaceous minerals (chlorite, biotite and illite – greens). [4] The color shifts from reddish to greenish as iron in the oxidized (ferric) state is converted to iron in the reduced (ferrous) state. [6] Black shale results from the presence of greater than one percent carbonaceous material and indicates a reducing environment. [4] Pale blue to blue-green shales typically are rich in carbonate minerals. [7]

Clays are the major constituent of shales and other mudrocks. The clay minerals represented are largely kaolinite, montmorillonite and illite. Clay minerals of Late Tertiary mudstones are expandable smectites, whereas in older rocks (especially in mid-to early Paleozoic shales) illites predominate. The transformation of smectite to illite produces silica, sodium, calcium, magnesium, iron and water. These released elements form authigenic quartz, chert, calcite, dolomite, ankerite, hematite and albite, all trace to minor (except quartz) minerals found in shales and other mudrocks. [4] A typical shale is composed of about 58% clay minerals, 28% quartz, 6% feldspar, 5% carbonate minerals, and 2% iron oxides. [8] Most of the quartz is detrital (part of the original sediments that formed the shale) rather than authigenic (crystallized within the shale after deposition). [9]

Shales and other mudrocks contain roughly 95 percent of the organic matter in all sedimentary rocks. However, this amounts to less than one percent by mass in an average shale. Black shales, which form in anoxic conditions, contain reduced free carbon along with ferrous iron (Fe2+) and sulfur (S2−). Amorphous iron sulfide, along with carbon, produce the black coloration. [4] Because amorphous iron sulfide gradually converts to pyrite, which is not an important pigment, young shales may be quite dark from their iron sulfide content, in spite of a modest carbon content (less than 1%), while a black color in an ancient shale indicates a high carbon content. [7]

Most shales are marine in origin, [10] and the groundwater in shale formations is often highly saline. There is evidence that shale acts as a semipermeable medium, allowing water to pass through while retaining dissolved salts. [11] [12]


The fine particles that compose shale can remain suspended in water long after the larger particles of sand have been deposited. As a result, shales are typically deposited in very slow moving water and are often found in lakes and lagoonal deposits, in river deltas, on floodplains and offshore below the wave base. [13] Thick deposits of shale are found near ancient continental margins [13] and foreland basins. [14] Some of the most widespread shale formations were deposited by epicontinental seas. Black shales [8] are common in Cretaceous strata on the margins of the Atlantic Ocean, where they were deposited in fault-bounded silled basins associated with the opening of the Atlantic during the breakup of Pangaea. These basins were anoxic, in part because of restricted circulation in the narrow Atlantic, and in part because the very warm Cretaceous seas lacked the circulation of cold bottom water that oxygenates the deep oceans today. [15]

Most clay must be deposited as aggregates and floccules, since the settling rate of individual clay particles is extremely slow. [16] Flocculation is very rapid once the clay encounters highly saline sea water. [17] Whereas individual clay particles are less than 4 microns in size, the clumps of clay particles produced by flocculation vary in size from a few tens of microns to over 700 microns in diameter. The floccules start out water-rich, but much of the water is expelled from the floccules as the clay minerals bind more tightly together over time (a process called syneresis). [18] Clay pelletization by organisms that filter feed is important where flocculation is inhibited. Filter feeders produce an estimated 12 metric tons of clay pellets per square kilometer per year along the U.S. Gulf Coast. [19]

As sediments continue to accumulate, the older, more deeply buried sediments begin to undergo diagenesis. This mostly consists of compaction and lithification of the clay and silt particles. [20] [21] Early stages of diagenesis, described as eogenesis, take place at shallow depths (a few tens of meters) and are characterized by bioturbation and mineralogical changes in the sediments, with only slight compaction. [22] Pyrite may be formed in anoxic mud at this stage of diagenesis. [8] [23]

Deeper burial is accompanied by mesogenesis, during which most of the compaction and lithification takes place. As the sediments come under increasing pressure from overlying sediments, sediment grains move into more compact arrangements, ductile grains (such as clay mineral grains) are deformed, and pore space is reduced. [24] In addition to this physical compaction, chemical compaction may take place via pressure solution. Points of contact between grains are under the greatest strain, and the strained mineral is more soluble than the rest of the grain. As a result, the contact points are dissolved away, allowing the grains to come into closer contact. [21]

It is during compaction that shale develops its fissility, likely through mechanical compaction of the original open framework of clay particles. The particles become strongly oriented into parallel layers that give the shale its distinctive fabric. [25] Fissility likely develops early in the compaction process, at relatively shallow depth, since fissility does not seem to vary with depth in thick formations. [26] Kaolinite flakes have less tendency to align in parallel layers than other clays, so kaolinite-rich clay is more likely to form nonfissile mudstone than shale. On the other hand, black shales often have very pronounced fissility (paper shales) due to binding of hydrocarbon molecules to the faces of the clay particles, which weakens the binding between particles. [27]

Lithification follows closely on compaction, as increased temperatures at depth hasten deposition of cement that binds the grains together. Pressure solution contributes to cementing, as the mineral dissolved from strained contact points is redeposited in the unstrained pore spaces. The clay minerals may be altered as well. For example, smectite is altered to illite at temperatures of about 55 to 200 °C (130 to 390 °F), releasing water in the process. [8] Other alteration reactions include the alteration of smectite to chlorite and of kaolinite to illite at temperatures between 120 and 150 °C (250 and 300 °F). [8] Because of these reactions, illite composes 80% of Precambrian shales, versus about 25% of young shales. [28]

Unroofing of buried shale is accompanied by telogenesis, the third and final stage of diagenesis. [22] As erosion reduces the depth of burial, renewed exposure to meteoric water produces additional changes to the shale, such as dissolution of some of the cement to produce secondary porosity. Pyrite may be oxidized to produce gypsum. [21]

Black shales are dark, as a result of being especially rich in unoxidized carbon. Common in some Paleozoic and Mesozoic strata, black shales were deposited in anoxic, reducing environments, such as in stagnant water columns. [8] Some black shales contain abundant heavy metals such as molybdenum, uranium, vanadium, and zinc. [8] [29] [30] [31] The enriched values are of controversial origin, having been alternatively attributed to input from hydrothermal fluids during or after sedimentation or to slow accumulation from sea water over long periods of sedimentation. [30] [32] [33]

Fossils, animal tracks or burrows and even raindrop impressions are sometimes preserved on shale bedding surfaces. Shales may also contain concretions consisting of pyrite, apatite, or various carbonate minerals. [34]

Shales that are subject to heat and pressure of metamorphism alter into a hard, fissile, metamorphic rock known as slate. With continued increase in metamorphic grade the sequence is phyllite, then schist and finally gneiss. [35]

As hydrocarbon source rock

Shale is the most common source rock for hydrocarbons (natural gas and petroleum). [8] The lack of coarse sediments in most shale beds reflects the absence of strong currents in the waters of the depositional basin. These might have oxygenated the waters and destroyed organic matter before it could accumulate. The absence of carbonate rock in shale beds reflects the absence of organisms that might have secreted carbonate skeletons, also likely due to an anoxic environment. As a result, about 95% of organic matter in sedimentary rocks is found in shales and other mudrocks. Individual shale beds typically have an organic matter content of about 1%, but the richest source rocks may contain as much as 40% organic matter. [36]

The organic matter in shale is converted over time from the original proteins, polysaccharides, lipids, and other organic molecules to kerogen, which at the higher temperatures found at greater depths of burial is further converted to graphite and petroleum. [37]

Historical mining terminology

Before the mid-19th century, the terms slate, shale and schist were not sharply distinguished. [38] In the context of underground coal mining, shale was frequently referred to as slate well into the 20th century. [39] Black shale associated with coal seams is called black metal. [40]

See also

Related Research Articles

<span class="mw-page-title-main">Limestone</span> Type of sedimentary rock

Limestone is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of CaCO3. Limestone forms when these minerals precipitate out of water containing dissolved calcium. This can take place through both biological and nonbiological processes, though biological processes, such as the accumulation of corals and shells in the sea, have likely been more important for the last 540 million years. Limestone often contains fossils which provide scientists with information on ancient environments and on the evolution of life.

<span class="mw-page-title-main">Sandstone</span> Type of sedimentary rock

Sandstone is a clastic sedimentary rock composed mainly of sand-sized silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.

<span class="mw-page-title-main">Clay</span> Fine grained soil

Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, Al2Si2O5(OH)4). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impurities, such as a reddish or brownish colour from small amounts of iron oxide.

<span class="mw-page-title-main">Slate</span> Metamorphic rock

Slate is a fine-grained, foliated, homogeneous, metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade, regional metamorphism. It is the finest-grained foliated metamorphic rock. Foliation may not correspond to the original sedimentary layering, but instead is in planes perpendicular to the direction of metamorphic compression.

<span class="mw-page-title-main">Sedimentary rock</span> Rock formed by the deposition and cementation of particles

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

<span class="mw-page-title-main">Chert</span> Hard, fine-grained sedimentary rock composed of cryptocrystalline silica

Chert is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood.

<span class="mw-page-title-main">Silt</span> Classification of soil or sediment

Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil or as sediment mixed in suspension with water. Silt usually has a floury feel when dry, and lacks plasticity when wet. Silt can also be felt by the tongue as granular when placed on the front teeth.

Sedimentology encompasses the study of modern sediments such as sand, silt, and clay, and the processes that result in their formation, transport, deposition and diagenesis. Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary structures.

<span class="mw-page-title-main">Concretion</span> Compact mass formed by precipitation of mineral cement between particles

A concretion is a hard, compact mass formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion' is derived from the Latin concretio "(act of) compacting, condensing, congealing, uniting", itself from con meaning "together" and crescere meaning "to grow".

<span class="mw-page-title-main">Lithology</span> Description of its physical characteristics of a rock unit

The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Examples of lithologies in the second sense include sandstone, slate, basalt, or limestone.

<span class="mw-page-title-main">Siltstone</span> Sedimentary rock which has a grain size in the silt range

Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.

<span class="mw-page-title-main">Clay mineral</span> Fine-grained aluminium phyllosilicates

Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2Si2O5(OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces.

<span class="mw-page-title-main">Mudstone</span> Fine grained sedimentary rock whose original constituents were clays or muds

Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from shale by its lack of fissility.

<span class="mw-page-title-main">Mudrock</span> Type of sedimentary rock

Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone and shale. Most of the particles of which the stone is composed are less than 116 mm and are too small to study readily in the field. At first sight, the rock types appear quite similar; however, there are important differences in composition and nomenclature.

<span class="mw-page-title-main">Clastic rock</span> Sedimentary rocks made of mineral or rock fragments

Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks, and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic to refer to sedimentary rocks and particles in sediment transport, whether in suspension or as bed load, and in sediment deposits.

<span class="mw-page-title-main">Fissility (geology)</span> Tendency of a rock to split along flat planes of weakness

In geology, fissility is the ability or tendency of a rock to split along flat planes of weakness. These planes of weakness are oriented parallel to stratification in sedimentary rocks. Fissility is differentiated from scaly fabric in hand sample by the parting surfaces’ continuously parallel orientations to each other and to stratification. Fissility is distinguished from scaly fabric in thin section by the well-developed orientation of platy minerals such as mica. Fissility is the result of sedimentary or metamorphic processes.

<span class="mw-page-title-main">Oil shale geology</span> Branch of geology

Oil shale geology is a branch of geologic sciences which studies the formation and composition of oil shales–fine-grained sedimentary rocks containing significant amounts of kerogen, and belonging to the group of sapropel fuels. Oil shale formation takes place in a number of depositional settings and has considerable compositional variation. Oil shales can be classified by their composition or by their depositional environment. Much of the organic matter in oil shales is of algal origin, but may also include remains of vascular land plants. Three major type of organic matter (macerals) in oil shale are telalginite, lamalginite, and bituminite. Some oil shale deposits also contain metals which include vanadium, zinc, copper, and uranium.

<span class="mw-page-title-main">Lamination (geology)</span> Thin layers present in sedimentary rock

In geology, lamination is a small-scale sequence of fine layers that occurs in sedimentary rocks. Laminae are normally smaller and less pronounced than bedding. Lamination is often regarded as planar structures one centimetre or less in thickness, whereas bedding layers are greater than one centimetre. However, structures from several millimetres to many centimetres have been described as laminae. A single sedimentary rock can have both laminae and beds.

<span class="mw-page-title-main">Shallow water marine environment</span>

Shallow water marine environment refers to the area between the shore and deeper water, such as a reef wall or a shelf break. This environment is characterized by oceanic, geological and biological conditions, as described below. The water in this environment is shallow and clear, allowing the formation of different sedimentary structures, carbonate rocks, coral reefs, and allowing certain organisms to survive and become fossils.

<span class="mw-page-title-main">Úrkút Manganese Ore Formation</span>

The Úrkút Manganese Ore Formation is a Jurassic geologic formation in Hungary. It covers the Early Toarcian stage of the Early Jurassic, and it is one of the main regional units linked to the Toarcian Anoxic Events. Different fossils heve been recovered on the locations, including marine life such as Ammonites Fish and terrestrial fossils, such as Palynomorphs and fossil wood. Úrkút and Eplény are the main deposits of the Formation. Are related to the Bakony Range, an ancient massif that was uplifted gradually and exposed to a long period of erosion, where the deposits of Úrkút appear to be a basin inclined gently to the north, while the highest point to the south is the basalt mass of Kab Mountain. Eplény region consists of a broad N-S trending open valley between fiat-topped, small hills.


  1. 1 2 Blatt, Harvey and Robert J. Tracy (1996) Petrology: Igneous, Sedimentary and Metamorphic, 2nd ed., Freeman, pp. 281–292 ISBN   0-7167-2438-3
  2. "Rocks: Materials of the Lithosphere – Summary". Archived from the original on 24 December 2014. Retrieved 2007-07-31.
  3. Boggs, Sam (2006). Principles of sedimentology and stratigraphy (4th ed.). Upper Saddle River, N.J.: Pearson Prentice Hall. p. 139. ISBN   0131547283.
  4. 1 2 3 4 5 6 Blatt, Harvey and Robert J. Tracy (1996) Petrology: Igneous, Sedimentary and Metamorphic, 2nd ed., Freeman, pp. 281–292 ISBN   0-7167-2438-3
  5. "Rocks: Materials of the Lithosphere – Summary". Archived from the original on 24 December 2014. Retrieved 2007-07-31.
  6. Potter, Paul Edwin; Maynard, J. Barry; Pryor, Wayne A. (1980). Sedimentology of shale : study guide and reference source. New York: Springer-Verlag. pp. 54–56. ISBN   0387904301.
  7. 1 2 Potter, Maynard & Pryor 1980, p. 56.
  8. 1 2 3 4 5 6 7 8 Ferriday, Tim; Montenari, Michael (2016). "Chemostratigraphy and Chemofacies of Source Rock Analogues: A High-Resolution Analysis of Black Shale Successions from the Lower Silurian Formigoso Formation (Cantabrian Mountains, NW Spain)". Stratigraphy & Timescales. 1: 123–255. doi:10.1016/bs.sats.2016.10.004 via Elsevier Science Direct.
  9. Potter, Maynard & Pryor 1980, pp. 47–49.
  10. Potter, Maynard & Pryor 1980, p. 72.
  11. Potter, Maynard & Pryor 1980, p. 59.
  12. Berry, F.A. (1960). "Geologic field evidence suggesting membrane properties of shales". AAPG Bulletin. 44 (6): 953–954. Retrieved 13 April 2021.
  13. 1 2 Blatt & Tracy 1996, p. 219.
  14. Fillmore, Robert (2010). Geological evolution of the Colorado Plateau of eastern Utah and western Colorado, including the San Juan River, Natural Bridges, Canyonlands, Arches, and the Book Cliffs. Salt Lake City: University of Utah Press. p. 222-223, 236-241. ISBN   9781607810049.
  15. Blatt & Tracy 1996, pp. 287–292.
  16. Potter, Maynard & Pryor 1980, p. 8.
  17. McCave, I.N. (1975). "Vertical flux of particles in the ocean". Deep Sea Research and Oceanographic Abstracts. 22 (7): 491–502. Bibcode:1975DSRA...22..491M. doi:10.1016/0011-7471(75)90022-4.
  18. Potter, Maynard & Pryor 1980, p. 9.
  19. Potter, Maynard & Pryor 1980, p. 10.
  20. Blatt & Tracy 1996, pp. 265–280.
  21. 1 2 3 Boggs 2006, pp. 147–154.
  22. 1 2 Choquette, P.W.; Pray, L.C. (1970). "Geologic Nomenclature and Classification of Porosity in Sedimentary Carbonates". AAPG Bulletin. 54. doi:10.1306/5D25C98B-16C1-11D7-8645000102C1865D.
  23. Boggs 2006, p. 148.
  24. Richardson, Ethan J.; Montenari, Michael (2020). "Assessing shale gas reservoir potential using multi-scaled SEM pore network characterizations and quantifications: The Ciñera-Matallana pull-apart basin, NW Spain". Stratigraphy & Timescales. 5: 677–755. doi:10.1016/bs.sats.2020.07.001. ISBN   9780128209912. S2CID   229217907 via Elsevier Science Direct.
  25. Lash, G. G.; Blood, D. R. (1 January 2004). "Origin of Shale Fabric by Mechanical Compaction of Flocculated Clay: Evidence from the Upper Devonian Rhinestreet Shale, Western New York, U.S.A.". Journal of Sedimentary Research. 74 (1): 110–116. Bibcode:2004JSedR..74..110L. doi:10.1306/060103740110.
  26. Sintubin, Manuel (1994). "Clay fabrics in relation to the burial history of shales". Sedimentology. 41 (6): 1161–1169. Bibcode:1994Sedim..41.1161S. doi:10.1111/j.1365-3091.1994.tb01447.x.
  27. Blatt, Harvey; Middleton, Gerard; Murray, Raymond (1980). Origin of sedimentary rocks (2d ed.). Englewood Cliffs, N.J.: Prentice-Hall. pp. 398–400. ISBN   0136427103.
  28. Boggs 2006, pp. 142, 145–154.
  29. R. Zangerl and E. S. Richardson (1963) The paleoecologic history of two Pennsylvanian shales, Fieldiana Memoirs v. 4, Field Museum of Natural History, Chicago
  30. 1 2 J.D. Vine and E.B. Tourtelot (1970). "Geochemistry of black shale deposits – A summary report". Economic Geology. 65 (3): 253–273. doi:10.2113/gsecongeo.65.3.253.
  31. R.M. Coveney (1979). "Zinc concentrations in mid-continent Pennsylvanian black shales of Missouri and Kansas". Economic Geology. 74: 131–140. doi:10.2113/gsecongeo.74.1.131.
  32. R.M. Coveney (2003) "Metalliferous Paleozoic black shales and associated strata" in D.R. Lenz (ed.) Geochemistry of Sediments and Sedimentary Rocks, Geotext 4, Geological Association of Canada pp. 135–144
  33. H.D. Holland (1979). "Metals in black shales – A reassessment". Economic Geology. 70 (7): 1676–1680. doi:10.2113/gsecongeo.74.7.1676.
  34. Potter, Maynard & Pryor 1980, pp. 22–23.
  35. Potter, Maynard & Pryor 1980, p. 14.
  36. Blatt, Middleton & Murray 1980, pp. 396–397.
  37. Blatt, Middleton & Murray 1980, pp. 397.
  38. R. W. Raymond (1881) "Slate" in A Glossary of Mining and Metallurigical Terms, American Institute of Mining Engineers. p. 78.
  39. Albert H. Fay (1920) "Slate" in A Glossary of the Mining and Mineral Industry, United States Bureau of Mines. p. 622.
  40. Herbert, Bucksch (1996). Dictionary geotechnical engineering: English German. Springer. p. 61. ISBN   978-3540581642.

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