Siltstone, also known as aleurolite, is a
clastic sedimentary rock
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 ...
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 when ...
. 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 content, which can be distinguished from
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
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
Tight gas is natural gas produced from reservoir rocks with such low permeability that massive hydraulic fracturing is necessary to produce the well at economic rates. This natural gas is trapped within rocks with very low permeability, in other ...
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 carbo ...
that requires
hydraulic fracturing for economic gas production.
Siltstone was prized in
ancient Egypt for manufacturing statuary and
cosmetic palette
Cosmetic palettes are archaeological Artifact (archaeology), artifacts, originally used in predynastic Egypt, predynastic ancient Egypt, Egypt to grind and apply ingredients for facial or body cosmetics. The decorative palettes of the late 4th mill ...
s. The siltstone quarried at
Wadi Hammamat
Wadi Hammamat ( en, Valley of Many Baths, ''India way; gateway to India'') is a dry river bed in Egypt's Eastern Desert, about halfway between Al-Qusayr and Qena. It was a major mining region and trade route east from the Nile Valley in ancie ...
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 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.
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
Field may refer to:
Expanses of open ground
* Field (agriculture), an area of land used for agricultural purposes
* Airfield, an aerodrome that lacks the infrastructure of an airport
* Battlefield
* Lawn, an area of mowed grass
* Meadow, a grass ...
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 silicates) ...
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 par ...
fraction. Although often mistaken for a
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
, siltstone lacks the laminations and
fissility along horizontal lines which are typical of shale.
Siltstones may contain
concretion
A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular ...
s. Unless the siltstone is fairly shaly,
stratification
Stratification may refer to:
Mathematics
* Stratification (mathematics), any consistent assignment of numbers to predicate symbols
* Data stratification in statistics
Earth sciences
* Stable and unstable stratification
* Stratification, or st ...
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 form ...
.
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 environment
In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be ...
s where fine particles can settle out of the transporting medium (air or water) and accumulate on the surface.
They are found in
turbidite
A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean.
Sequencing
Turbidites were ...
sequences, in deltas, in glacial deposits, and in miogeosynclinal settings.
Locations with Siltstone donation
*
Cheltenham Badlands
The Cheltenham Badlands are in Caledon, Ontario, on the southeast side of Olde Base Line Road, between Creditview and Chinguacousy Roads. The site occupies an area of approximately 0.4 square kilometers and features exposed and highly eroded Que ...
,
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 tot ...
*
Chek Chau
Port Island or Chek Chau () is an island of Hong Kong, under the administration of Tai Po District. It is located in Tolo Channel, in the northeastern New Territories. Its name literally means red island; after the sedimentary rocks rich in ...
,
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 city and special administrative region of China on the eastern Pearl River Delt ...
- 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