Conglomerate (/kənˈɡlɒmərət/) is asedimentary rock made up of roundedgravel-sized pieces of rock surrounded by finer-grainedsediments (such assand,silt, orclay). The larger fragments within conglomerate are called clasts, while the finer sediment surrounding the clasts is called the matrix. The clasts and matrix are typically cemented bycalcium carbonate,iron oxide,silica, or hardened clay.
Conglomerates form when rounded gravels deposited by water or glaciers becomesolidified and cemented by pressure over time. 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. They are closely related to sandstones in origin, and exhibit many of the same types ofsedimentary structures, such as tabular and troughcross-bedding andgraded bedding.[1][2][3]
Fanglomerates are poorly sorted, matrix-rich conglomerates that originated asdebris flows onalluvial fans and likely contain the largest accumulations of gravel in the geologic record.[4]
Breccias are similar to conglomerates, but have clasts that have angular (rather than rounded) shapes.
The classification method depends on the type and detail of research being conducted.[1][2][5]
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 aconglomerate. If the gravel clasts that comprise it are largely angular, it is abreccia. 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.[2][5]
Conglomerates contain at least 30% of rounded to subangular clasts larger than 2 mm (0.079 in) in diameter, e.g.,granules,pebbles,cobbles, andboulders. 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 asmatrix. 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 andfloat 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 eithercalcite,hematite,quartz, or clay.[1][2][5]
The differences between paraconglomerates and orthoconglomerates reflect differences in how they are deposited. Paraconglomerates are commonly either glacialtills ordebris flow deposits. Orthoconglomerates are typically associated with aqueous currents.[1][2][5]
A conglomerate at the base of theCambrian in the Black Hills, South Dakota.Section of polymict conglomerate from offshore rockcore,Alaska, approximate depth 10,000 ft.
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.[2][3][6]
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.[2][3][6]
Two recognized types of intraformational conglomerates are shale-pebble and flat-pebble conglomerates.[6] 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.[7] 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.[8]
Finally, conglomerates are often differentiated and named according to the dominantclast 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.[5][6]
Inturbidites, 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 typeimbrication of the clasts.[9]
Conglomerates are normally present at the base of sequences laid down duringmarine transgressions above anunconformity, and are known asbasal conglomerates. They represent the position of theshoreline at a particular time and arediachronous.[10]
Conglomerates deposited in fluvial environments are typically well rounded and poorly sorted. Clasts of this size are carried asbedload and only at times of high flow-rate. The maximum clast size decreases as the clasts are transported further due toattrition, 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 aspebble lags.[11] Conglomerates deposited in a fluvial environment often have an AB-plane type imbrication.
Alluvial deposits form in areas of high relief and are typically coarse-grained. At mountain fronts individualalluvial 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-planeimbrication. 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) environments, the resulting rock unit is often called afanglomerate.[9]
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 aseskers.[11]
An example of conglomerate can be seen atMontserrat, nearBarcelona. 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 theSanta Maria de Montserrat Abbey.
Another example, theCrestone Conglomerate, occurs in and near the town ofCrestone, at the foot of theSangre de Cristo Range inColorado'sSan Luis Valley. 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.
OnMars, slabs of conglomerate have been found at anoutcrop named "Hottah", and have been interpreted by scientists as having formed in an ancient streambed. The gravels, which were discovered byNASA's Mars roverCuriosity, 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.[16]
^abcdBoggs, S. (2006)Principles of Sedimentology and Stratigraphy., 2nd ed. Prentice Hall, New York. 662 pp.ISBN0-13-154728-3
^abcdefgFriedman, 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.ISBN978-1-4020-0872-6
^abcdNeuendorf, 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.ISBN0-922152-76-4
^Leeder, Mike (2011).Sedimentology and sedimentary basins : from turbulence to tectonics (2nd ed.). Chichester, West Sussex, UK: Wiley-Blackwell. p. 290.ISBN9781405177832.
^abcdeNichols, G. (2009)Sedimentology and Stratigraphy, 2nd ed. John Wiley & Sons Ltd, Chichester, West Sussex, United Kingdom. 419 pp.ISBN978-1-4051-9379-5
^Williams, G. D. (1966)Origin of Shale-Pebble Conglomerate. American Association of Petroleum Geologist Bulletin. vol. 50, no. 3, pp. 573–577.
^Flugel, E. (2010)Microfacies of Carbonate Rocks: Analysis, Interpretation and Application, 2nd ed. Springer-Verlag, Berlin, Germany.ISBN978-3-642-03795-5
^abWalker, R. G. 1979. Facies Models. Reprinted with revisions from a series of papers in Geoscience Canada, 1976–1979, Geological Association of Canada
^Bourman, Robert P.; Ollier, Clifford D.; Buckman, Solomon (2015-06-01). "Inselbergs and monoliths: a comparative review of two iconic Australian landforms, Uluru (Ayers Rock) and Burringurrah (Mount Augustus)".Zeitschrift für Geomorphologie.59 (2):197–227.Bibcode:2015ZGm....59..197B.doi:10.1127/0372-8854/2014/0148.ISSN0372-8854.
^Neuendorf, K.K.E.; Mehl, Jr., J.P.; Jackson, J.A., eds. (2005).Glossary of Geology (5th ed.). Alexandria, Virginia: American Geological Institute. p. 431.ISBN978-0922152896.
_30.jpg)
