Conglomerate is a clastic sedimentary rock that shaped from rounded gravel and boulder sized clasts cemented or in a matrix supperted. The rounding of the clasts show that rocks have been transported a long way from their source or on a seaside tide to wave movement. The clast cement is usually calcite, silica or iron oxide but the matrix can consist only of the cementing cloth, however can also include sand and / or silt sized clasts cemented together the various coarser clasts.

Class: Conglomerate may be divided into large lessons:

Texture: Clastic (coarse-grained).

Grain size: > 2mm; Clasts easily visible to the naked eye, should be identifiable.

Hardness: Soft to hard, dependent on clast composition and strength of cement.

Colour: variable, dependent on clast and matrix composition.

Clasts: variable, but generally harder rock types and / or minerals dominate.

Other features: Clasts generally smooth to touch, matrix variable.

Classification of Conglomerate

Conglomerate Rock well-rounded clasts

Conglomerates named and classifield by the

  1. Type and amount of matrix present
  2. Composition of gravel-size clasts they contain
  3. Size range of gravel-size clasts present

A sedimentary rock consisting mainly of gravel is first named according to the roundness of the gravel. If the gravel clasts that form it are well-rounded to subrounded, to a large extent, it is a conglomerate. If the pebble clips forming it are largely angular, it is a breccia. Such breccias may be called sedimentary breccias to distinguish them from other breccia types.

  1. The amount and chemical composition of the matrix. If the clasts do not touch each other (lots of matrix), the rock is paraconglomerate. Rock in which the clasts touch each other is called orthoconglomerate.
  2. The composition of the clasts. If all the clasts are the same type of rock or mineral), the rock is categorized as monomictic conglomerate. If the clasts are made up of two or more rocks or minerals, the rock is a polymictic conglomerate.
  3. The size of the clasts. Rock comprised of large clasts is cobble conglomerate. If the clasts are pebble-sized, the rock is called pebble conglomerate. If the clasts are small granules, the rock is called granule conglomerate.

The environment that deposited the material. Conglomerates may form from glacial, alluvial, fluvial, deepwater marine, or shallow marine environments.

Conglomerate Composition

Conglomerate is a type of sedimentary rock that is composed primarily of rounded or water-worn pebbles, cobbles, and boulders, which are known as clasts. These clasts are typically cemented together by a matrix of finer-grained sedimentary material, such as sand, silt, or clay. The composition of conglomerate can vary widely depending on the source of the clasts and the type of cementing material, but here are the main components:

  1. Clasts: The clasts in conglomerate rocks can be made up of a variety of materials, including:
    • Rock fragments: These can include pebbles, cobbles, and boulders of different types of rocks, such as granite, limestone, sandstone, shale, or even volcanic rocks like basalt.
    • Mineral fragments: In addition to rock fragments, conglomerates may contain mineral fragments that have been transported and rounded by water or other agents.
  2. Matrix: The matrix is the fine-grained material that fills the spaces between the clasts and cements them together. The matrix can consist of:
    • Sand: When the matrix is primarily composed of sand-sized particles, the rock is sometimes called a “sandstone conglomerate.”
    • Silt: If the matrix is dominated by silt-sized particles, it may be referred to as a “siltstone conglomerate.”
    • Clay: In some cases, the matrix can be clay-rich, leading to a “claystone conglomerate.”
  3. Cement: The cementing material is responsible for binding the clasts together and hardening the rock. Common cementing agents in conglomerate include:
    • Silica (silica cement): Silica, in the form of minerals like quartz, can precipitate from pore fluids and bind the clasts together.
    • Calcium carbonate (calcite cement): In some cases, calcium carbonate can act as the cementing material, especially in areas with abundant limestone.
    • Iron oxide (hematite or limonite cement): Iron oxides can also cement clasts together, giving the rock a reddish or yellowish hue.

The specific composition of conglomerate rocks can vary widely based on the geological history of the area where they formed and the type of sediments available for deposition. Conglomerates are typically associated with high-energy environments like rivers, alluvial fans, or coastal areas where the clasts are transported and deposited by water or gravity. Over time, the sediments are compacted and cemented together to form conglomerate rock.

Formation and Occurrence

Conglomerate rocks form through a specific process of sedimentary deposition and lithification (the process of turning sediments into solid rock). They are typically associated with high-energy environments and can be found in various geological settings. Here’s how conglomerates form and where they commonly occur:

Formation Process:

  1. Transportation: The formation of conglomerate begins with the transportation of large clasts (pebbles, cobbles, and boulders) by agents like rivers, streams, alluvial fans, or glaciers. These agents have the energy to move and round the clasts over long distances.
  2. Deposition: When the transporting agents lose their energy (e.g., when a river slows down or a glacier melts), they deposit the clasts along with finer-grained sedimentary material like sand, silt, or clay.
  3. Sorting: Conglomerates often exhibit poor sorting, meaning the clasts can vary in size and composition. This is because the energy of the transporting agent may not be sufficient to sort the clasts by size or type.
  4. Cementation: Over time, as the sediment accumulates, the clasts become buried under additional layers of sediment. The weight and pressure from overlying sediments force the water out of the pore spaces between the clasts.
  5. Cementing: As the pore spaces are squeezed out, minerals like silica, calcium carbonate, or iron oxides can precipitate from groundwater and fill the gaps between the clasts. This cementing process binds the clasts together, hardening the sediment into rock.

Common Occurrences of Conglomerates:

  1. Riverbeds and Alluvial Fans: Conglomerates are frequently found in riverbeds, where the high-energy flow of water can transport and deposit a variety of clasts. Alluvial fans, which form at the base of mountain ranges and result from the rapid deposition of sediment by flowing water, are also common locations for conglomerates.
  2. Coastal Environments: Coastal areas with strong wave action and tides can lead to the accumulation of conglomerate deposits. The clasts in coastal conglomerates are often rounded and well-polished due to the abrasive action of the sea.
  3. Glacial Environments: Glaciers can transport and deposit large amounts of rock and sediment, including conglomerates, as they move and retreat.
  4. Fault Zones: In some cases, fault zones can create conditions for the formation of conglomerates. Faulting can bring together rocks of different types and sizes, leading to the deposition of conglomerate material along fault lines.
  5. Ancient Alluvial Plains: In the geological record, conglomerates are often found in ancient alluvial plains where rivers once flowed, deposited sediments, and eventually turned them into rock.
  6. Mountainous Regions: Conglomerates can be exposed in mountainous regions through erosion and uplift processes. They may be found in sedimentary layers that were once buried but have since been exposed by tectonic forces.

Conglomerate rocks provide valuable information to geologists about the geological history and environmental conditions of the past. They can contain clues about the type and origin of the clasts, the energy of the depositional environment, and the age of the rock layer in which they are found.

Conglomerate Localities

Conglomerates are deposited in various 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.

Characteristics and Properties

Conglomerate is a distinctive sedimentary rock with several characteristic features and properties that help geologists identify and understand it. Here are the main characteristics and properties of conglomerate:

  1. Clastic Texture: Conglomerate has a clastic texture, which means it is composed of fragments or clasts that have been transported and deposited. These clasts are typically rounded and well-worn, although angular clasts can also be present, especially in immature conglomerates.
  2. Clast Composition: The composition of the clasts within conglomerate can vary widely. They may be made of different types of rocks, minerals, or even fossils, depending on the geological history of the area. Common clast types include granite, limestone, sandstone, shale, and volcanic rocks.
  3. Poor Sorting: Conglomerates often exhibit poor sorting, meaning that the clasts vary in size and may not be well-sorted by size or type. This is due to the variable energy levels of the transporting agents.
  4. Matrix: Conglomerate typically contains a matrix, which is a finer-grained material that fills the spaces between the clasts and cements them together. The matrix can consist of sand, silt, or clay, depending on the specific type of conglomerate.
  5. Cementation: The clasts in conglomerate are held together by a cementing material, which can include minerals like silica (quartz), calcium carbonate (calcite), or iron oxides (hematite or limonite). The cement helps harden the rock over time.
  6. Color: Conglomerate can come in a variety of colors, depending on the types of clasts and matrix materials present. It can range from red or brown to gray, green, or even black.
  7. Strength: Conglomerate is generally a strong and durable rock due to the cementation of clasts. It can resist weathering and erosion better than unconsolidated sediments.
  8. Fossil Preservation: In some cases, conglomerate can preserve fossils. Fossils may be found within the clasts or in the matrix material. Fossil-bearing conglomerates can provide valuable information about ancient ecosystems and environments.
  9. Stratification: Conglomerate layers often display a stratified appearance. This stratification results from the deposition of sediments in distinct layers or beds, with variations in clast size, sorting, or composition between layers.
  10. High Energy Environments: Conglomerate is typically associated with high-energy environments, such as riverbeds, alluvial fans, coastal areas, or glacial deposits. These environments have the energy to transport and deposit coarse clasts.
  11. Sedimentary Structures: Conglomerates may exhibit various sedimentary structures, including cross-bedding, imbrication (overlapping of clasts in a specific direction), and graded bedding. These structures provide insights into the flow dynamics and depositional history of the sediment.
  12. Age Indicators: Conglomerate layers in the geological record can be used as age indicators. They may contain fossils or be found in stratigraphic sequences that help date the rock and determine the geological history of an area.

Overall, conglomerate is a fascinating sedimentary rock that reflects the dynamic processes of sediment transport, deposition, and lithification. Its varied characteristics and properties provide valuable information to geologists about the geological history and environmental conditions of the past.

Conglomerate Uses and Application

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.

Conglomerate and Breccia

Conglomerates and breccias are two sedimentary rocks close to each other, but differ significantly in the form of clasts. Clasts in the conglomerate are rounded or at least partially rounded, whereas the clast in the breccias have sharp corners. Sometimes sedimentary rocks contain a mixture of round and angled buckles. This type of rock can be called breccio-conglomerate.

Facts

  • Conglomerate is closely related to sandstone and displays many of the same types of sedimentary structures. Sandstone is a notably popular building material, used for things like flagstones and tile.
  • Conglomerate rocks are colorful and attractive; however, it is rarely used as ornamental stone for interior use because of its unreliable physical strength and durability.
  • Conglomerate has very few commercial uses, though it can be crushed to make a fine aggregate that can be used when a low-performance material is needed.
  • Conglomerate forms where sediments of rounded clasts at least two millimeters in diameter accumulate. Because of the large size of the clasts, it takes a very strong water current to transport and shape the rocks. As they tumble through the running water or moving waves, they form their rounded shape.
  • These rocks can be found in sedimentary rock sequences of all ages. They probably make up less than one percent by weight of all sedimentary rocks.
  • When the gravel clasts in a conglomerate are separated from each other and contain more matrix than clasts, it is called a paraconglomerate. When they are in contact with each other, it is called a orthoconglomerate.
  • Similar sedimentary rocks that are composed of large angular clasts are referred to as breccia. While a conglomerate is composed of rounded clasts, breccia is composed of broken rocks or minerals.
  • NASA’s Mars rover Curiosity discovered an outcrop of conglomerate on the surface of Mars in September 2012. This provided evidence to scientists that a stream once ran across the area where the rover was driving. The shape and sizes of the stones can offer clues to the distance and speed of the stream’s flow.

References

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