Siltstone
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Siltstone, also known as aleurolite,<ref>Gyöngyi Farkas Characterization of subterranean bacteria in the Hungarian Upper Permian Siltstone (Aleurolite) Formation Canadian Journal of Microbiology 46(6):559-64</ref> 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.Template:Sfn
Although its permeability and porosity is relatively low, siltstone is sometimes a tight gas reservoir rock,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> an unconventional reservoir for natural gas that requires hydraulic fracturing for economic gas production.<ref>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.</ref>
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.<ref>Template:Cite book</ref>
DescriptionEdit
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.<ref name="folk-1965-145">Template:Cite book</ref> An alternate definition is that siltstone is any sedimentary rock containing 50% or more of silt-sized particles.<ref name="picard-1971">Template:Cite journal</ref> Siltstones can be distinguished from claystone in the field by chewing a small sample; claystone feels smooth while siltstone feels gritty.Template:Sfn
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.Template:Sfn Siltstones may contain concretions.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Unless the siltstone is fairly shaly, stratification is likely to be obscure and it tends to weather at oblique angles unrelated to bedding.
OriginEdit
Siltstone is an unusual rock, in which most of the silt grains are made of quartz.<ref name="nahon">Template:Cite journal</ref> The origin of quartz silt has been a topic of much research and debate.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Some quartz silt likely has its origin in fine-grained foliated metamorphic rock,Template:Sfn while much marine silt is likely biogenic,<ref>Template:Cite book</ref><ref>Template:Cite journal</ref> but most quartz sediments come from granitic rocks in which quartz grains are much larger than quartz silt.<ref>Template:Cite book</ref> Highly energetic processes are required to break these grains down to silt size.<ref name="assallay">Template:Cite journal</ref> Among proposed mechanism are glacial grinding;<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> weathering in cold, tectonically active mountain ranges;<ref name="assallay"/> normal weathering, particularly in tropical regions;<ref name="nahon"/><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> and formation in hot desert environments by salt weathering.<ref>Template:Cite journal</ref>
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.<ref name="lillie-2005">Template:Cite book</ref> They are found in turbidite sequences,<ref>Jaworowski, K. (2013). Facies analysis of the Silurian shale-siltstone succession in Pomerania (northern Poland). Geological Quarterly, 44(3), 297-315. Retrieved from https://gq.pgi.gov.pl/article/view/8078</ref> in deltas,<ref>Lineback, Jerry Alvin. "Deep-water sediments adjacent to the Borden Siltstone (Mississippian) delta in southern Illinois." Circular no. 401 (1966).</ref> in glacial deposits,<ref>Template:Cite journal</ref> and in miogeosynclinal settings.<ref>Template:Cite journal</ref>
Locations with siltstone donationEdit
- Cheltenham Badlands, Canada
- Chek Chau, Hong Kong - Siltstone layered with conglomerate
FootnotesEdit
ReferencesEdit
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- Williams, Howel, Francis J. Turner and Charles M. Gilbert, 1954, Petrography, W. H. Freeman