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Sedimentary rock

Siltstone, also known as aleurolite, [1] 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. [2]


Although its permeability and porosity is relatively low, siltstone is sometimes a tight gas reservoir rock, [3] [4] an unconventional reservoir for natural gas that requires hydraulic fracturing for economic gas production. [5]

Siltstone was prized in ancient Egypt for manufacturing statuary and cosmetic palettes. The siltstone quarried at Wadi Hammamat was a hard, fine-grained siltstone that resisted flaking and was almost ideal for such uses. [6]


Holtzclaw siltstone, Louisville, Kentucky Holtzclaw siltstone-Kentucky.jpg
Holtzclaw siltstone, Louisville, Kentucky

There is not complete agreement on the definition of siltstone. One definition is that siltstone is mudrock (clastic sedimentary rock containing at least 50% clay and silt) in which at least 2/3 of the clay and silt fraction is composed of silt-sized particles. Silt is defined as grains 2–62  μm in diameter, or 4 to 8 on the Krumbein phi (φ) scale. [7] An alternate definition is that siltstone is any sedimentary rock containing 50% or more of silt-sized particles. [8] Siltstones can be distinguished from claystone in the field by chewing a small sample; claystone feels smooth while siltstone feels gritty. [2]

Siltstones differ significantly from sandstones due to their smaller pores and a higher propensity for containing a significant clay fraction. Although often mistaken for a shale, siltstone lacks the laminations and fissility along horizontal lines which are typical of shale. [2] Siltstones may contain concretions. [9] [10] Unless the siltstone is fairly shaly, stratification is likely to be obscure and it tends to weather at oblique angles unrelated to bedding.


Siltstone is an unusual rock, in which most of the silt grains are made of quartz. [11] The origin of quartz silt has been a topic of much research and debate. [12] [13] Some quartz silt likely has its origin in fine-grained foliated metamorphic rock, [14] while much marine silt is likely biogenic, [15] [16] but most quartz sediments come from granitic rocks in which quartz grains are much larger than quartz silt. [17] Highly energetic processes are required to break these grains down to silt size. [18] Among proposed mechanism are glacial grinding; [19] [20] weathering in cold, tectonically active mountain ranges; [18] normal weathering, particularly in tropical regions; [11] [21] [22] and formation in hot desert environments by salt weathering. [23]

Siltstones form in relatively quiet depositional environments where fine particles can settle out of the transporting medium (air or water) and accumulate on the surface. [24] They are found in turbidite sequences, [25] in deltas, [26] in glacial deposits, [27] and in miogeosynclinal settings. [28]

Locations with Siltstone donation


  1. Gyöngyi Farkas Characterization of subterranean bacteria in the Hungarian Upper Permian Siltstone (Aleurolite) Formation Canadian Journal of Microbiology 46(6):559-64
  2. 1 2 3 Blatt et al. 1980, pp.381-382
  3. Clarkson, Christopher R.; Jensen, Jerry L.; Pedersen, Per Kent; Freeman, Melissa (February 2012). "Innovative methods for flow-unit and pore-structure analyses in a tight siltstone and shale gas reservoir". AAPG Bulletin. 96 (2): 355–374. doi:10.1306/05181110171.
  4. Cao, Zhe; Liu, Guangdi; Zhan, Hongbin; Gao, Jin; Zhang, Jingya; Li, Chaozheng; Xiang, Baoli (May 2017). "Geological roles of the siltstones in tight oil play". Marine and Petroleum Geology. 83: 333–344. doi:10.1016/j.marpetgeo.2017.02.020.
  5. Ben E. Law and Charles W. Spencer, 1993, "Gas in tight reservoirs-an emerging major source of energy," in David G. Howell (ed.), The Future of Energy Gasses, US Geological Survey, Professional Paper 1570, p.233-252.
  6. Shaw, Ian (2004). Ancient Egypt : a very short introduction. Oxford: Oxford University Press. pp. 44–45. ISBN   0192854194 . Retrieved 2 October 2020.
  7. Folk, R.L. (1980). Petrology of sedimentary rocks (2nd ed.). Austin: Hemphill's Bookstore. p. 145. ISBN   0-914696-14-9. Archived from the original on 2006-02-14. Retrieved 2 October 2020.
  8. Picard, M. Dane (1971). "Classification of Fine-grained Sedimentary Rocks". SEPM Journal of Sedimentary Research. 41. doi:10.1306/74D7221B-2B21-11D7-8648000102C1865D.
  9. Melezhik, Victor A.; Fallick, Anthony E.; Smith, Richard A.; Rosse, Danta M. (December 2007). "Spherical and columnar, septarian, 18 O-depleted, calcite concretions from Middle–Upper Permian lacustrine siltstones in northern Mozambique: evidence for very early diagenesis and multiple fluids". Sedimentology. 54 (6): 1389–1416. doi:10.1111/j.1365-3091.2007.00886.x.
  10. Middleton, Heather A.; Nelson, Campbell S. (May 1996). "Origin and timing of siderite and calcite concretions in late Palaeogene non- to marginal-marine facies of the Te Kuiti Group, New Zealand". Sedimentary Geology. 103 (1–2): 93–115. doi:10.1016/0037-0738(95)00092-5.
  11. 1 2 Nahon, D.; Trompette, R. (February 1982). "Origin of siltstones: glacial grinding versus weathering". Sedimentology. 29 (1): 25–35. doi:10.1111/j.1365-3091.1982.tb01706.x.
  12. Nemecz, Ernö; Pécsi, Márton; Hartyáni, Zsuzsa; Horváth, Timea (June 2000). "The origin of the silt size quartz grains and minerals in loess". Quaternary International. 68–71: 199–208. doi:10.1016/S1040-6182(00)00044-6.
  13. Smalley, Ian (January 1990). "Possible formation mechanisms for the modal coarse-silt quartz particles in loess deposits". Quaternary International. 7–8: 23–27. doi:10.1016/1040-6182(90)90035-3.
  14. Blatt et al. 1980, p.284
  15. Leeder, M. R. (2011). Sedimentology and sedimentary basins : from turbulence to tectonics (2nd ed.). Chichester, West Sussex, UK: Wiley-Blackwell. ISBN   9781405177832.
  16. Schieber, Jürgen; Krinsley, Dave; Riciputi, Lee (August 2000). "Diagenetic origin of quartz silt in mudstones and implications for silica cycling". Nature. 406 (6799): 981–985. doi:10.1038/35023143. PMID   10984049. S2CID   4417951.
  17. Potter, Paul Edwin; Maynard, James; Pryor, Wayne A. (1980). Sedimentology of shale : study guide and reference source. New York: Springer-Verlag. ISBN   0387904301.
  18. 1 2 Assallay, A (November 1998). "Silt: 2–62 μm, 9–4φ". Earth-Science Reviews. 45 (1–2): 61–88. doi:10.1016/S0012-8252(98)00035-X.
  19. Kuenen, P. H. (1 December 1969). "Origin of quartz silt". Journal of Sedimentary Research. 39 (4): 1631–1633. doi:10.1306/74D71ED3-2B21-11D7-8648000102C1865D.
  20. Riezebos, P.A.; Van der Waals, L. (December 1974). "Silt-sized quartz particles: a proposed source". Sedimentary Geology. 12 (4): 279–285. doi:10.1016/0037-0738(74)90022-0.
  21. Iriondo, Martı́n (December 1999). "The origin of silt particles in the loess question". Quaternary International. 62 (1): 3–9. doi:10.1016/S1040-6182(99)00018-X.
  22. Pye, Kenneth (April 1983). "Formation of quartz silt during humid tropical weathering of dune sands". Sedimentary Geology. 34 (4): 267–282. doi:10.1016/0037-0738(83)90050-7.
  23. Goudie, A.S.; Cooke, R.U.; Doornkamp, J.C. (June 1979). "The formation of silt from quartz dune sand by salt-weathering processes in deserts". Journal of Arid Environments. 2 (2): 105–112. doi:10.1016/S0140-1963(18)31786-5.
  24. Lillie, Robert J. (2005). Parks and plates : the geology of our national parks, monuments, and seashores (1st ed.). New York: W.W. Norton. ISBN   0393924076.
  25. Jaworowski, K. (2013). Facies analysis of the Silurian shale-siltstone succession in Pomerania (northern Poland). Geological Quarterly, 44(3), 297-315. Retrieved from
  26. Lineback, Jerry Alvin. "Deep-water sediments adjacent to the Borden Siltstone (Mississippian) delta in southern Illinois." Circular no. 401 (1966).
  27. Thomas, S. G.; Fielding, C. R.; Frank, T. D. (December 2007). "Lithostratigraphy of the late Early Permian (Kungurian) Wandrawandian Siltstone, New South Wales: record of glaciation?". Australian Journal of Earth Sciences. 54 (8): 1057–1071. doi:10.1080/08120090701615717. S2CID   46570542.
  28. Ethridge, F.G. (1977). "Petrology, Transport, and Environment in Isochronous Upper Devonian Sandstone and Siltstone Units, New York". SEPM Journal of Sedimentary Research. 47. doi:10.1306/212F70EF-2B24-11D7-8648000102C1865D.

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Limestone Sedimentary rocks made of calcium carbonate

Limestone is a common type of carbonate sedimentary rock. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate. 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 have likely been more important for the last 540 million years. Limestone often contains fossils, and these provide scientists with information on ancient environments and on the evolution of life.

Sandstone 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.

Shale Fine-grained, clastic sedimentary rock

Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. Shale is characterized by its tendency to split into thin layers (laminae) less than one centimeter in thickness. This property is called fissility. Shale is the most common sedimentary rock.

Sedimentary rock Rock formed by the deposition and subsequent cementation of material

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.

Marl Lime-rich mud or mudstone which contains variable amounts of clays and silt

Marl or marlstone is a carbonate-rich mud or mudstone which contains variable amounts of clays and silt. The term was originally 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. These typically contain 35–65% clay and 65–35% carbonate. The term is today often used to describe indurated marine deposits and lacustrine (lake) sediments which more accurately should be named 'marlstone'.

Sediment Particulate solid matter that is deposited on the surface of land

Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation; if buried, they may eventually become sandstone and siltstone through lithification.

Chert 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.

Gravel Mix of crumbled stones: grain size range between 2 – 63 mm according to ISO 14688

Gravel is a loose aggregation of rock fragments. Gravel is classified by particle size range and includes size classes from granule- to boulder-sized fragments. In the Udden-Wentworth scale gravel is categorized into granular gravel and pebble gravel. ISO 14688 grades gravels as fine, medium, and coarse, with ranges 2–6.3 mm to 20–63 mm. One cubic metre of gravel typically weighs about 1,800 kg.

Silt Classification of soil or sediment

Silt is granular material of a size between sand and clay, whose mineral origin is quartz and feldspar. Silt may occur as a soil or as sediment mixed in suspension with water and soil in a body of water such as a river. It may also exist as soil deposited at the bottom of a water body, like mudflows from landslides. Silt has a moderate specific area with a typically non-sticky, plastic feel. Silt usually has a floury feel when dry, and a slippery feel when wet. Silt can be visually observed with a hand lens, exhibiting a sparkly appearance. It also can be felt by the tongue as granular when placed on the front teeth.

Loess Sediment of accumulated wind-blown dust

Loess is a clastic, predominantly silt-sized sediment that is formed by the accumulation of wind-blown dust. Ten percent of Earth's land area is covered by loess or similar deposits.

Concretion Compact mass formed by precipitation of mineral cement between particles

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Mudstone 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.

Mudrock Class of fine grained siliciclastic sedimentary rocks

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 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.

Red beds Sedimentary rocks with ferric oxides

Red beds are sedimentary rocks, typically consisting of sandstone, siltstone, and shale, that are predominantly red in color due to the presence of ferric oxides. Frequently, these red-colored sedimentary strata locally contain thin beds of conglomerate, marl, limestone, or some combination of these sedimentary rocks. The ferric oxides, which are responsible for the red color of red beds, typically occur as a coating on the grains of sediments comprising red beds. Classic examples of red beds are the Permian and Triassic strata of the western United States and the Devonian Old Red Sandstone facies of Europe.

Clastic rock Sedimentary rocks made of mineral or rock fragments

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Syneresis crack

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Grain size Diameter of individual grains of sediment, or of lithified particles in clastic rocks

Grain size is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which refers to the size of a single crystal inside a particle or grain. A single grain can be composed of several crystals. Granular material can range from very small colloidal particles, through clay, silt, sand, gravel, and cobbles, to boulders.

Provenance in geology, is the reconstruction of the origin of sediments. The Earth is a dynamic planet, and all rocks are subject to transition between the three main rock types: sedimentary, metamorphic, and igneous rocks. Rocks exposed to the surface are sooner or later broken down into sediments. Sediments are expected to be able to provide evidence of the erosional history of their parent source rocks. The purpose of provenance study is to restore the tectonic, paleo-geographic and paleo-climatic history.

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