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Conglomerate () is a
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 that is composed of a substantial fraction of rounded to subangular
gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classifi ...
-size clasts. A conglomerate typically contains a
matrix Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** '' The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchi ...
of finer-grained sediments, such as sand, silt, or
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 part ...
, which fills the interstices between the clasts. The clasts and matrix are typically cemented by calcium carbonate,
iron oxide Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of whic ...
, silica, or hardened clay. Conglomerates form by the
consolidation Consolidation may refer to: In science and technology * Consolidation (computing), the act of linkage editing in computing * Memory consolidation, the process in the brain by which recent memories are crystallised into long-term memory * Pulmon ...
and lithification of gravel. They can be found in sedimentary rock sequences of all ages but probably make up less than 1 percent by weight of all sedimentary rocks. In terms of origin and depositional mechanisms, they are closely related to sandstones and exhibit many of the same types of sedimentary structures, e.g., tabular and trough cross-bedding and graded bedding.Boggs, S. (2006) ''Principles of Sedimentology and Stratigraphy.'', 2nd ed. Prentice Hall, New York. 662 pp. Friedman, G.M. (2003) ''Classification of sediments and sedimentary rocks.'' In Gerard V. Middleton, ed., pp. 127-135, ''Encyclopedia of Sediments & Sedimentary Rocks, Encyclopedia of Earth Science Series.'' Kluwer Academic Publishers, Boston, Massachusetts. 821 pp. Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) ''Glossary of Geology'' (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. Fanglomerates are poorly sorted, matrix-rich conglomerates that originated as debris flows on alluvial fans and likely contain the largest accumulations of gravel in the geologic record.


Classification of conglomerates

Conglomerates may be named and classified by the: * Amount and type of matrix present * Composition of gravel-size clasts they contain * Size range of gravel-size clasts present The classification method depends on the type and detail of research being conducted.Nichols, G. (2009) ''Sedimentology and Stratigraphy,'' 2nd ed. John Wiley & Sons Ltd, Chichester, West Sussex, United Kingdom. 419 pp. A sedimentary rock composed largely of gravel is first named according to the roundness of the gravel. If the gravel clasts that comprise it are largely well-rounded to subrounded, it is a ''conglomerate.'' If the gravel clasts that comprise it are largely angular, it is a breccia. Such breccias can be called sedimentary breccias to differentiate them from other types of breccia, e.g. volcanic and fault breccias. Sedimentary rocks that contain a mixture of rounded and angular gravel clasts are sometimes called breccio-conglomerate.


Texture

Conglomerates contain at least 30% of rounded to subangular clasts larger than in diameter, e.g.,
granule A granule is a large particle or grain. It can refer to: * Granule (cell biology), any of several submicroscopic structures, some with explicable origins, others noted only as cell type-specific features of unknown function ** Azurophilic granul ...
s,
pebble A pebble is a clast of rock with a particle size of based on the Udden-Wentworth scale of sedimentology. Pebbles are generally considered larger than granules ( in diameter) and smaller than cobbles ( in diameter). A rock made predominant ...
s, cobbles, and
boulder In geology, a boulder (or rarely bowlder) is a rock fragment with size greater than in diameter. Smaller pieces are called cobbles and pebbles. While a boulder may be small enough to move or roll manually, others are extremely massive. In c ...
s. However, conglomerates are rarely composed entirely of gravel-size clasts. Typically, the space between the gravel-size clasts is filled by a mixture composed of varying amounts of silt, sand, and clay, known as ''matrix''. If the individual gravel clasts in a conglomerate are separated from each other by an abundance of matrix such that they are not in contact with each other and ''float'' within the matrix, it is called a paraconglomerate. Paraconglomerates are also often unstratified and can contain more matrix than gravel clasts. If the gravel clasts of a conglomerate are in contact with each other, it is called an orthoconglomerate. Unlike paraconglomerates, orthoconglomerates are typically cross-bedded and often well-cemented and lithified by either
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
,
hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...
, quartz, or clay. The differences between paraconglomerates and orthoconglomerates reflect differences in how they are deposited. Paraconglomerates are commonly either glacial tills or debris flow deposits. Orthoconglomerates are typically associated with aqueous currents.


Clast composition

Conglomerates are also classified according to the composition of their clasts. A conglomerate or any clastic sedimentary rock that consists of a single rock or mineral is known as either a monomict, monomictic, oligomict, or oligomictic conglomerate. If the conglomerate consists of two or more different types of rocks, minerals, or combination of both, it is known as either a polymict or polymictic conglomerate. If a polymictic conglomerate contains an assortment of the clasts of metastable and unstable rocks and minerals, it is called either a petromict or petromictic conglomerate. Tucker, M. E. (2003) ''Sedimentary Rocks in the Field'', 3rd ed. John Wiley & Sons Ltd,West Sussex, England. In addition, conglomerates are classified by source as indicated by the lithology of the gravel-size clasts If these clasts consist of rocks and minerals that are significantly different in lithology from the enclosing matrix and, thus, older and derived from outside the basin of deposition, the conglomerate is known as an extraformational conglomerate. If these clasts consist of rocks and minerals that are identical to or consistent with the lithology of the enclosing matrix and, thus, penecontemporaneous and derived from within the basin of deposition, the conglomerate is known as an intraformational conglomerate. Two recognized types of intraformational conglomerates are shale-pebble and flat-pebble conglomerates. A shale-pebble conglomerate is a conglomerate that is composed largely of clasts of rounded mud chips and pebbles held together by clay minerals and created by erosion within environments such as within a river channel or along a lake margin.Williams, G. D. (1966) ''Origin of Shale-Pebble Conglomerate.'' American Association of Petroleum Geologist Bulletin. vol. 50, no. 3, pp. 573–577. Flat-pebble conglomerates (edgewise conglomerates) are conglomerates that consist of relatively flat clasts of lime mud created by either storms or tsunami eroding a shallow sea bottom or tidal currents eroding tidal flats along a shoreline.Flugel, E. (2010) ''Microfacies of Carbonate Rocks: Analysis, Interpretation and Application'', 2nd ed. Springer-Verlag, Berlin, Germany.


Clast size

Finally, conglomerates are often differentiated and named according to the dominant clast size comprising them. In this classification, a conglomerate composed largely of granule-size clasts would be called a granule conglomerate; a conglomerate composed largely of pebble-size clasts would be called a pebble conglomerate; and a conglomerate composed largely of cobble-size clasts would be called a cobble conglomerate.


Sedimentary environments

Conglomerates are deposited in a variety of sedimentary environments.


Deepwater marine

In turbidites, the basal part of a bed is typically coarse-grained and sometimes conglomeratic. In this setting, conglomerates are normally very well sorted, well-rounded and often with a strong A-axis type
imbrication Imbrication is the arrangement of planar bodies such that they stack in a consistent fashion - rather like a toppled run of dominoes. *In roofing, imbrication is employed in the Imbrex and tegula system. *Imbrication (sedimentology) In sedime ...
of the clasts.Walker, R. G. 1979. Facies Models. Reprinted with revisions from a series of papers in Geoscience Canada, 1976–1979, Geological Association of Canada


Shallow marine

Conglomerates are normally present at the base of sequences laid down during marine transgressions above an unconformity, and are known as basal conglomerates. They represent the position of the shoreline at a particular time and are diachronous.


Fluvial

Conglomerates deposited in fluvial environments are typically well rounded and poorly sorted. Clasts of this size are carried as bedload and only at times of high flow-rate. The maximum clast size decreases as the clasts are transported further due to attrition, so conglomerates are more characteristic of immature river systems. In the sediments deposited by mature rivers, conglomerates are generally confined to the basal part of a channel fill where they are known as ''pebble lags''. Tucker, M. E.br>''Sedimentary petrology''
3rd edition, 2001, WileyBlackwell
Conglomerates deposited in a fluvial environment often have an AB-plane type imbrication.


Alluvial

Alluvial deposits form in areas of high relief and are typically coarse-grained. At mountain fronts individual alluvial fans merge to form braidplains and these two environments are associated with the thickest deposits of conglomerates. The bulk of conglomerates deposited in this setting are clast-supported with a strong AB-plane
imbrication Imbrication is the arrangement of planar bodies such that they stack in a consistent fashion - rather like a toppled run of dominoes. *In roofing, imbrication is employed in the Imbrex and tegula system. *Imbrication (sedimentology) In sedime ...
. Matrix-supported conglomerates, as a result of debris-flow deposition, are quite commonly associated with many alluvial fans. When such conglomerates accumulate within an alluvial fan, in rapidly eroding (e.g.,
desert A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to denudation. About o ...
) environments, the resulting rock unit is often called a ''fanglomerate''.


Glacial

Glaciers carry a lot of coarse-grained material and many glacial deposits are conglomeratic. tillites, the sediments deposited directly by a glacier, are typically poorly sorted, matrix-supported conglomerates. The matrix is generally fine-grained, consisting of finely milled rock fragments. Waterlaid deposits associated with glaciers are often conglomeratic, forming structures such as eskers.


Examples

An example of conglomerate can be seen at
Montserrat Montserrat ( ) is a British Overseas Territory in the Caribbean. It is part of the Leeward Islands, the northern portion of the Lesser Antilles chain of the West Indies. Montserrat is about long and wide, with roughly of coastline. It is ni ...
, near
Barcelona Barcelona ( , , ) is a city on the coast of northeastern Spain. It is the capital and largest city of the autonomous community of Catalonia, as well as the second most populous municipality of Spain. With a population of 1.6 million within ...
. Here, erosion has created vertical channels that give the characteristic jagged shapes the mountain is named for (Montserrat literally means "jagged mountain"). The rock is strong enough to use as a building material, as in the Santa Maria de Montserrat Abbey. Another example, the Crestone Conglomerate, occurs in and near the town of Crestone, at the foot of the Sangre de Cristo Range in
Colorado Colorado (, other variants) is a state in the Mountain states, Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the wes ...
's
San Luis Valley The San Luis Valley is a region in south-central Colorado with a small portion overlapping into New Mexico. The valley is approximately long and wide, extending from the Continental Divide on the northwest rim into New Mexico on the south. It co ...
. The Crestone Conglomerate consists of poorly sorted fanglomerates that accumulated in prehistoric alluvial fans and related fluvial systems. Some of these rocks have hues of red and green. Conglomerate cliffs are found on the east coast of Scotland from
Arbroath Arbroath () or Aberbrothock ( gd, Obar Bhrothaig ) is a former royal burgh and the largest town in the council area of Angus, Scotland, with a population of 23,902. It lies on the North Sea coast some ENE of Dundee and SSW of Aberdeen. The ...
northwards along the coastlines of the former counties of
Angus Angus may refer to: Media * ''Angus'' (film), a 1995 film * ''Angus Og'' (comics), in the ''Daily Record'' Places Australia * Angus, New South Wales Canada * Angus, Ontario, a community in Essa, Ontario * East Angus, Quebec Scotland * Angu ...
and Kincardineshire. Dunnottar Castle sits on a rugged promontory of conglomerate jutting into the North Sea just south of the town of Stonehaven.
Copper Harbor Conglomerate The Copper Harbor Conglomerate is a geologic formation in Michigan. It is part of the larger Oronto Group and its formation dates to the Stenian period of the Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2 ...
is found both in the
Keweenaw Peninsula The Keweenaw Peninsula ( , sometimes locally ) is the northernmost part of Michigan's Upper Peninsula. It projects into Lake Superior and was the site of the first copper boom in the United States, leading to its moniker of "Copper Country." As o ...
and Isle Royale National Park in Lake Superior. Conglomerate may also be seen in the domed hills of Kata Tjuta, in Australia's Northern Territory or in the Buda Hills in Hungary. In the nineteenth century a thick layer of
Pottsville Pottsville usually refers to the city of Pottsville, Pennsylvania, in the United States. Pottsville may also refer to: Other communities *Pottsville, New South Wales, Australia *Pottsville, Arkansas, United States *Pottsville, Kentucky, United ...
conglomerate was recognized to underlie
anthracite coal Anthracite, also known as hard coal, and black coal, is a hard, compact variety of coal that has a submetallic luster. It has the highest carbon content, the fewest impurities, and the highest energy density of all types of coal and is the high ...
measures in Pennsylvania.


Examples on Mars

On Mars, slabs of conglomerate have been found at an outcrop named " Hottah", and have been interpreted by scientists as having formed in an ancient streambed. The gravels, which were discovered by NASA's Mars rover Curiosity, range from the size of sand particles to the size of golf balls. Analysis has shown that the pebbles were deposited by a stream that flowed at walking pace and was ankle- to hip-deep.


Metaconglomerate

Metamorphic alteration transforms conglomerate into metaconglomerate.


See also

* Puddingstone *
Jasper conglomerate Jasper conglomerate is an informal term for a very distinctive Paleoproterozoic quartz and jasper pebble conglomerate that occurs within the middle part of the Lorrain Formation of the Cobalt Group of the Huronian Supergroup. It is also known by o ...


References


External links


Conglomerate at Cushendun, Northern Ireland
{{Webarchive, url=https://web.archive.org/web/20160303200906/http://www.geographyinaction.co.uk/Assets/Photo_albums/Seven/pages/Conglomerate.html , date=2016-03-03 Sedimentary rocks