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Sedimentary rock

Siltstone 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. [1]


Although its permeability and porosity is relatively low, siltstone is sometimes a tight gas reservoir rock, [2] [3] an unconventional reservoir for natural gas that requires hydraulic fracturing for economic gas production. [4]

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. [5]


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. [6] An alternate definition is that siltstone is any sedimentary rock containing 50% or more of silt-sized particles. [7] Siltstones can be distinguished from claystone away from the laboratory by chewing a small sample; claystone feels smooth while siltstone feels gritty. [1]

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. [1] Siltstones may contain concretions. [8] [9] 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. [10] The origin of quartz silt has been a topic of much research and debate. [11] [12] Some quartz silt likely has its origin in fine-grained foliated metamorphic rock, [13] while much marine silt is likely biogenic, [14] [15] , but most quartz sediments come from granitic rocks in which quartz grains are much larger than quartz silt. [16] Highly energetic processes are required to break these grains down to silt size. [17] Among proposed mechanism are glacial grinding; [18] [19] weathering in cold, tectonically active mountain ranges; [17] normal weathering, particularly in tropical regions; [10] [20] [21] and formation in hot desert environments by salt weathering. [22]

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. [23] They are found in turbidite sequences, [24] in deltas, [25] , in glacial deposits, [26] and in miogeosynclinal settings. [27]


  1. 1 2 3 Blatt et al. 1980, pp.381-382
  2. 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.
  3. 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.
  4. 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.
  5. Shaw, Ian (2004). Ancient Egypt : a very short introduction. Oxford: Oxford University Press. pp. 44–45. ISBN   0192854194 . Retrieved 2 October 2020.
  6. Folk, R.L. (1980). Petrology of sedimentary rocks (2nd ed.). Austin: Hemphill's Bookstore. p. 145. ISBN   0-914696-14-9 . Retrieved 2 October 2020.
  7. Picard, M. Dane (1971). "Classification of Fine-grained Sedimentary Rocks". SEPM Journal of Sedimentary Research. Vol. 41. doi:10.1306/74D7221B-2B21-11D7-8648000102C1865D.
  8. 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.
  9. 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.
  10. 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.
  11. 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.
  12. 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.
  13. Blatt et al. 1980, p.284
  14. Leeder, M. R. (2011). Sedimentology and sedimentary basins : from turbulence to tectonics (2nd ed.). Chichester, West Sussex, UK: Wiley-Blackwell. ISBN   9781405177832.
  15. 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.
  16. Potter, Paul Edwin; Maynard, James; Pryor, Wayne A. (1980). Sedimentology of shale : study guide and reference source. New York: Springer-Verlag. ISBN   0387904301.
  17. 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.
  18. 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.
  19. 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.
  20. 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.
  21. 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.
  22. 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.
  23. 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.
  24. Jaworowski, K. (2013). Facies analysis of the Silurian shale-siltstone succession in Pomerania (northern Poland). Geological Quarterly, 44(3), 297-315. Retrieved from
  25. Lineback, Jerry Alvin. "Deep-water sediments adjacent to the Borden Siltstone (Mississippian) delta in southern Illinois." Circular no. 401 (1966).
  26. 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.
  27. Ethridge, F.G. (1977). "Petrology, Transport, and Environment in Isochronous Upper Devonian Sandstone and Siltstone Units, New York". SEPM Journal of Sedimentary Research. Vol. 47. doi:10.1306/212F70EF-2B24-11D7-8648000102C1865D.

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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'.

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.

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  2. It is difficult to study mudrock constituents, due to their diminutive size and susceptibility to weathering on outcrops
  3. And most importantly, there is more than one classification scheme accepted by scientists
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