Conglomerate Rock Type, Composition, Formation, Classification, Uses

Conglomerate Rock
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The conglomerate is an example of a typical sedimentary rock. It is a clastic sedimentary rock usually composed of rounded clasts larger than 2 millimeters in diameter. The spaces between these clasts are usually filled with sand and clay particles. Like other typical sedimentary rocks, the rock is held together with calcite or quartz mineral cement.

The conglomerate is tan to brown and can be deposited in different sedimentary environments. The rocks’ properties and composition vary from one environment to another. Its texture is coarse-grained, and the clasts range from 2-64 millimeters.

Boulder of conglomerate with cobble-sized clast
Boulder of conglomerate with cobble-sized clast

Conglomerate Composition

Conglomerate’s composition varies based on the clasts of rock material it contains or weathering product that came downstream.

However, most conglomerate specimens contain mineral particles such as quartz, feldspar, or fragments from other sedimentary, igneous or metamorphic rocks. Conglomerates with clasts of sandstone, quartzite, limestone, granite, basalt, and gneiss are common.

A mixture of sand, mud, or chemical cement binds these fragments or clasts together. The common chemical cement is calcite or quartz.

Conglomerate Formation

Conglomerate forms as a result of gravel consolidation and lithification. Conglomerates are found in sedimentary rock sequences of all ages but comprise less than 1% of the weight of all sedimentary rocks. Conglomerates are closely related to sandstones as far as origin and deposition are concerned.

Conglomerates form at sites with deposits of rounded sediment clasts that are at least 2 millimeters in diameter. The deposition environment might be along a flowing stream or beach with strong waves. This may only happen during extreme flows or wave action.

Conglomerate formation requires a source of large-size sediment particles up current. Its rounded shape reveals that the clasts were rolled over from quite some distance by waves or running water. This environment is usually found in lakes, streams, and oceans.

Conglomerates can be deposited in various sedimentary environments.

Deepwater marine: Conglomerates formed in deepwater settings are well sorted and rounded and often have a strong A-axis type imbrication of the clasts.

Shallow marine: These are known as basal conglomerates as well. These conglomerates represent the position of the shoreline at a specific time and can be diachronous.

Fluvial: These are well-rounded but poorly sorted. These rocks often have an AB-plane type imbrication.

Alluvial: These conglomerates form in areas of high relief. These rocks are typically coarse-grained and have a strong AB-plane imbrication.

Glacial: Many glacial deposits are conglomeratic. Glacial till is sediment deposited by glaciers and is a poorly sorted and matrix-supported conglomerate.

A conglomerate at the base of the Cambrian in the Black Hills, South Dakota.
A conglomerate at the base of the Cambrian in the Black Hills, South Dakota.

Conglomerate Classification

Conglomerates are classified on the following three properties.

  • Type and amount of matrix
  • Class composition in the rock
  • Class size range

Conglomerate vs. Breccia

Conglomerate and breccia are often compared together. These rocks look similar but are different in various details.

The biggest similarity is that both have clasts bigger than two millimeters in diameter. The biggest difference is the shape of the clasts. Breccia has subangular or angular clasts, whereas conglomerate has subrounded or rounded clasts.

Sedimentary clasts get rounded if a stream or action of waves transports the clasts. They become angular if chemical and physical weathering act on the rock and break it into smaller pieces.

Where is Conglomerate Found?

Conglomerate reserves are located worldwide. Some notable reserves in the world are mentioned below.

  • Montserrat, Barcelona
  • San Luis Valley, Colorado
  • Scotland
  • Copper Harbor Conglomerate, Michigan
  • Kata Tjuta, Australia
  • Buda Hills, Hungary
Fanglomerate in Death Valley National Park
Fanglomerate in Death Valley National Park

Conglomerate on Mars

Congolermate’s presence has been reported on Mars as well. It has been found at an outcrop named “Hottah.” The gravels range from the size of sand to golf balls. NASA’s Mars Curiosity Rover discovered it in September 2012.

Conglomerate Uses

A conglomerate’s failure to break cleanly limits its use as a dimensional stone. It has a variable composition that gives the rock variable strength and durability, limiting its use in heavy-duty construction projects. However, it is used in projects that require low-performance material.

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