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Siltstone

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Siltstone
Sedimentary rock
Siltstone from California

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 production to be economical.[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]

Description

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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.

Origin

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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

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Footnotes

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  1. ^ Gyöngyi Farkas Characterization of subterranean bacteria in the Hungarian Upper Permian Siltstone (Aleurolite) Formation Canadian Journal of Microbiology 46(6):559-64
  2. ^ a b c Blatt, Middleton & Murray 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. Bibcode:2012BAAPG..96..355C. 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. Bibcode:2017MarPG..83..333C. 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. ^