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]

Description [ edit ]

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 [ edit ]

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]

Footnotes [ edit ]

  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 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. ^ a b 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. ^ a b 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.

References [ edit ]

  • Blatt, Harvey; Middleton, Gerard; Murray, Raymond (1980). Origin of sedimentary rocks (2d ed.). Prentice-Hall. ISBN 0136427103.
  • Williams, Howel, Francis J. Turner and Charles M. Gilbert, 1954, Petrography, W. H. Freeman
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