What is conglomerate?
Conglomerate is a clastic sedimentary rock that is formed by the accumulation of rock fragments greater than 2 millimetre in size. The rock fragments that forms the conglomerate are round in shape. The pore spaces in between the clast of a rock are filled by the finer particles usually silt and clay or any other cementing material that binds the rock fragments together.
Conglomerate can be formed of any rock material that is transported through time and space from the origin area to accumulation area. The rounded rock fragments is the proof of transportation of the rock fragments in the conglomerate. The rock fragments of the conglomerate can be derived from igneous, metamorphic or sedimentary rocks. The chemical binding material can be sand, silt or chemical cement like calcite.
What is the Composition of Conglomerate?
Conglomerate can have a variety of compositions. As a clastic sedimentary rock, it can contain clasts of any rock material or weathering product that is washed downstream or down current. The rounded clasts of conglomerate can be mineral particles such as quartz, or they can be sedimentary, metamorphic, or igneous rock fragments. The matrix that binds the large clasts together can be a mixture of sand, mud, and chemical cement.
Generation of conglomerate
Conglomerate can be formed at an areas where strong water current exist like mountain down slope where water has enough current flow that it can carry the rock fragments above 2 millimetre. It can also be formed at beaches where water current is strong and rock fragments are available to be accumulated for forming conglomerate. Conglomerate is formed when large clast pebble or cobble size fragments transported and deposited than the finer grained fills the spaces in between the clast.
Classification of conglomerate
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.
A sedimentary rock composed largely of gravel is first named according to the roundness of the gravel. If the gravel clasts that comprise it is 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 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 a orthoconglomerate. Unlike paraconglomerates, orthoconglomerates are typically cross-bedded and often well-cemented and lithified by either calcite, hematite, 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 tyipically associated with aqueous currents of some sort.
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 called either a petromict or petromictic conglomerate.
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 type 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. 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
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.
Conglomerate forming 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 of the clasts.
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 well 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. 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. 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 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 of conglomerate
An example of conglomerate can be seen at Montserrat, near Barcelona. 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's San 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.
Conglomerate cliffs are found on the east coast of Scotland from Arbroath northwards along the coastlines of the former counties of Angus and Kincardineshire. Dunottar Castle sits on a rugged promontory of conglomerate jutting into the North Sea just south of the town of Stonehaven.
Conglomerate may also be seen in the domed hills of Kata Tjuta, in Australia's Northern Territory.
In the nineteenth century a thick layer of Pottsville conglomerate was recognised to underlie anthracite coal measures in Pennsylvania.
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| This image was acquired by NASA's Curiosity rover on the surface of Mars. It shows an outcrop of conglomerate and some pebble-size weathering debris. The round pebbles are too large to have been moved and shaped by wind, thus they had to have been transported a significant distance by water. This photo from September 2012 was the strongest evidence of the existence of water on Mars that had been obtained at that time. |
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.
Uses of conglomerate
Conglomerate has very few uses because of it not clean breakage and fine particles are unreliable. It can only be used as a crush where low performance material is wanted. Conglomerate has very few commercial uses. Its inability to break cleanly makes it a poor candidate for dimension stone, and its variable composition makes it a rock of unreliable physical strength and durability. Conglomerate can be crushed to make a fine aggregate that can be used where a low-performance material is suitable. Many conglomerates are colorful and attractive rocks, but they are only rarely used as an ornamental stone for interior use.
Analysis of conglomerate can sometimes be used as a prospecting tool. For example, most diamond deposits are hosted in kimberlite. If a conglomerate contains clasts of kimberlite, then the source of that kimberlite must be somewhere upstream.
