Siltstone, also known as aleurolite, is a
clastic sedimentary rock that is composed mostly of
silt
Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel ...
. It is a form of
mudrock
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 and are too ...
with a low
clay mineral
Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces.
Clay minera ...
content, which can be distinguished from
shale by its lack of
fissility.
[Blatt ''et al.'' 1980, pp.381-382]
Although its permeability and porosity is relatively low, siltstone is sometimes a
tight gas reservoir rock, an
unconventional reservoir for
natural gas
Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon d ...
that requires
hydraulic fracturing for economic gas production.
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.
Description
There is not complete agreement on the definition of siltstone. One definition is that siltstone is mudrock (
clastic
Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...
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
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...
in diameter, or 4 to 8 on the
Krumbein phi (φ) scale.
An alternate definition is that siltstone is any sedimentary rock containing 50% or more of silt-sized particles.
Siltstones can be distinguished from claystone in the
field by chewing a small sample; claystone feels smooth while siltstone feels gritty.
Siltstones differ significantly from
sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicat ...
s due to their smaller pores and a higher propensity for containing a significant
clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay pa ...
fraction. Although often mistaken for a
shale, siltstone lacks the laminations and
fissility along horizontal lines which are typical of shale.
Siltstones may contain
concretions. Unless the siltstone is fairly shaly,
stratification is likely to be obscure and it tends to
weather
Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the ...
at oblique angles unrelated to bedding.
Origin
Siltstone is an unusual rock, in which most of the silt grains are made of
quartz
Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical f ...
.
The origin of quartz silt has been a topic of much research and debate. Some quartz silt likely has its origin in fine-grained foliated metamorphic rock,
[Blatt ''et al.'' 1980, p.284] while much marine silt is likely biogenic, but most quartz sediments come from granitic rocks in which quartz grains are much larger than quartz silt. Highly energetic processes are required to break these grains down to silt size.
Among proposed mechanism are glacial grinding; weathering in cold, tectonically active mountain ranges;
normal weathering, particularly in tropical regions;
and formation in hot desert environments by salt weathering.
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.
They are found in
turbidite sequences, in deltas, in glacial deposits, and in miogeosynclinal settings.
Locations with Siltstone donation
*
Cheltenham Badlands,
Canada
Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by to ...
*
Chek Chau,
Hong Kong
Hong Kong ( (US) or (UK); , ), officially the Hong Kong Special Administrative Region of the People's Republic of China (abbr. Hong Kong SAR or HKSAR), is a List of cities in China, city and Special administrative regions of China, special ...
- Siltstone layered with conglomerate
Footnotes
References
* {{cite book , last1=Blatt , first1=Harvey , last2=Middleton , first2=Gerard , last3=Murray , first3=Raymond , title=Origin of sedimentary rocks , date=1980 , publisher=Prentice-Hall , isbn=0136427103 , edition=2d
* Williams, Howel, Francis J. Turner and Charles M. Gilbert, 1954, ''Petrography'', W. H. Freeman
Sedimentary rocks