Did You Know?
In Australia, the Jack Hills located in the western regions is a classic example of metaconglomerate outcrops. These layers contain one of the oldest zircon crystals on our planet, which have been established to have embedded in the strata more than 4 billion years ago!
Formed in regions having abundant supply of water from bodies like lakes, rivers, sea waves, and even glaciers, conglomerates are classified under sedimentary rocks. They have been deposited in layers due to the action of few transporting and eroding mediums like water, glacier, wind, etc. These rocks are characterized by the presence of round to sub-angular-shaped mineral clasts that have been embedded in the matrix. Clasts are defined as the mineral grains that have been altered due to the above-described mediums, and have been transported over a long distance, resulting in them getting embedded in a fine-grained cement called matrix. The clasts can be of any type of rock, i.e., igneous, metamorphic, or sedimentary. The cement is mostly calcareous or siliceous in nature (containing maximum quantity of calcium carbonate and silica, respectively). Sometimes, the cement or matrix may also consist of clay particles, carbonaceous particles, gypsum fragments, iron oxide, mica flakes, feldspar, etc.
Texture, Classification, and Types
Though normally said to have coarse-grained texture, geologically speaking, conglomerates show clastic texture. This indicates that they have formed due to the deposition of sediments spanning across thousands of years. A graded texture can also be seen in the rocks.
Classification According to Percentage of Cement
This rock type mainly consists of more than 15% matrix of the total rock composition. The clasts mostly do not touch each other, and appear like they are floating in the matrix. Thus, this variety can also be called a matrix-supported conglomerate. In the given example, the light brown-colored mineral clasts of quartz are less as compared to the content of the calcareous cement, thus, making it a paraconglomerate. They are divided into laminated and non-laminated paraconglomerates. The former consists of mineral grains embedded in a laminated matrix. Main examples of this type are dropstones. The latter consist of clasts that are present in a non-laminated matrix. Main examples of this type are tillites (sediments transported by glacial action) and tilloids (sediments deposited due to debris flows).
On the contrary, orthoconglomerates are those that consist of less than 15% of matrix content, and hence are also known as clast-supported conglomerate rocks. For example, the image provided above shows numerous large-sized mineral grains that are embedded in less content of matrix, thus, making it an orthoconglomerate. The sides of the clasts are also touching each other, especially towards the right side portion of the sample. These rocks are further subdivided into petromict and oligomict orthoconglomerates.
Classification According to Resistance of Clasts
The conglomerates that consist of clasts that are not very much resistant to erosion and can be easily weathered are known as petromict conglomerates. For example, in such rocks, the clasts present are of limestone, dolomite, shale, etc.
The rocks that show the presence of high-resistance clasts that are not very susceptible to weathering and erosion are known as oligomict conglomerates. For example, mineral grains that are made up of basalt, quartz, chert, etc., are found in these rocks.
Based on Clast and Cement Composition
• Intraformational Conglomerates
These are characterized by the presence of clasts that almost have the same composition as that of the cement. This indicates that the mineral grains have originated from the same environment of deposition, or close to it, with regards to the area where the cement was formed. For example: quartz and jasper clasts embedded in a siliceous matrix.
• Extraformational Conglomerates
These rocks are characterized by the presence of clasts that have a very different composition as that of the cement or matrix. This indicates that they have formed in a different environment and were transported to a completely different area, where the cement was formed. For example: basalt clasts embedded in calcareous matrix.
Classification Based on Size and Composition
The mineral grains that are embedded in these rocks are mainly classified according to their size and composition. The size may vary, thus, imparting different degrees of coarseness and fineness to the rock. The minerals also show different colors, and are mostly visible as gray, yellow, light to dark brown, black, red, etc.
According to Grain Size
clast size greater than 200 mm
clast size between 20-200 mm
clast size between 6-20 mm
clast size between 2-6 mm
According to Most Common Clast Size
Alternatively, these rocks can also be classified by another way, which takes into account the most common clast size found in different samples. Thus, these categories are:
clast size greater than 256 mm
clast size in the range of 64-256 mm
clast size between 4-64 mm
clast size between 2-4 mm
According to Variation in Parent Material
The composition of clasts of these rocks depends upon the similarity or differences between them and the matrix, as well on the variation in the parent material among themselves.
• Monomictic Conglomerate
A rock that shows just one type of clast is called monomictic conglomerate. For example, in the accompanying image, the perfectly round-shaped and gray-colored clasts are of the same parent rock, and hence, they may form a uniformity in the arrangement of such clasts.
• Diamictic Conglomerate
In a rock, when the clasts are made up of few (at least 2-4) types of parent rocks, it is known as a diamictic conglomerate. For example, in the accompanying image, clasts made out of quartz and jasper can be seen embedded in the matrix.
• Polymictic Conglomerate
These conglomerates are characterized by the presence of numerous types of clasts, i.e., the minerals are made from a number of host or parent rocks. For example, in the accompanying image, the clasts are made up of basalt, quartz, limestone, chert, etc., and are embedded in the calcareous matrix.
The metamorphism of conglomerates leads to the alignment of the mineral clasts in a single direction, depending upon the intensity of the changes in temperature and/or pressure. In this type of sedimentary rock, the mineral grains may become deformed or might just break or shatter into small pieces, in case the degree of metamorphism is quite high. Sometimes, recrystallization of cementing material takes place, be it silicious or calcareous in nature. The subsequent products might include an inter-growth texture between the clasts just like that of a low-metamorphosed sandstone converted to a marble. If clay is present in the cement, then a small degree of slate formation might take place, along with foliated lamination interspersed with the aligned clasts.
For example, in the above image, the clasts are elongated and stretched in a single direction due to the layers undergoing cataclastic metamorphism. This is also known as a stretched conglomerate. The white- to off-white-colored clasts are round to sub-angular in shape, and are aligned in the brown-colored siliceous matrix. Also, the degree of metamorphism decreases from left to right, indicating variation in the changes of pressure and temperature that were acting on the embedded clasts after their formation.
Origin and Depositional Environment
The conglomeratic rocks are deposited in different types of environment, in both freshwater and marine conditions. The latter are mainly shoreline deposits, which are a part of turbidite sequences and basal conglomerates. The former are deposited on land by various mediums like rivers, glaciers, lakes, wind, etc. Such types are also formed during mountain building processes. Whatever the environment, the cementation of clasts takes several years, and during this time, the interstitial spaces between grains are slowly filled by the matrix.
• Shallow Water Marine Environment
Also known as basal conglomerates, these rocks formed in such conditions are usually found at the base of a certain rock formation; hence, the typical name. They usually form an unconformity, wherein the marine transgression that took place during the period of non-deposition is represented by the conglomerate layer.
• Deep Water Marine Environment
The sequences found in benthic conditions are known as turbidite sequences. These are characterized by the appearance of coarse-grained clasts, which show a well-sorted nature. The mineral grains also consist of a high degree of roundness, which indicates that they have been transported over long distances.
• Glacial Environment
The sedimentary fragments that are transported by glacial action often get cemented to form conglomerates, mainly near the glacier mouth. These rocks are mostly coarse-grained in nature, though few fine-grained varieties are seen. The deposits are poorly sorted in paraconglomerates. Tillites and eskers are the main types of such deposits found in this environment.
• Alluvial Environment
The conglomerates formed in such regions are a mixture of both orthoconglomerates and paraconglomerates. They are mostly coarse-grained in nature, and are also called fanglomerates. These rocks are also deposited beyond the shoreline, where they come in contact with numerous remains of plants and animals and form a part of oil fields.
• Fluvial Environment
The conglomerates formed in this environment show a good level of sorting and roundness, indicating strong water currents. The transported clasts undergo a process called saltation, wherein due to the immense force of water and wind, the grains 'jump' or roll off quickly on the river bed, or on the ground surface. The lacustrine environment conglomerates usually show graded bedding in the sedimentary deposits.
Major Examples of Conglomerate Outcrops
• In Spain, several hills and mountains are present near Barcelona, which are made up of beautiful columns of weathered conglomerate. These rocks are often used for construction purposes in those areas.
• In the San Luis Valley of Colorado, the Crestone deposits are a series of metaconglomerates. The mineral grains show various degrees of alignment, and are derived from different types of rocks.
• Other spectacular examples of these rocks are seen in Northern Australia, Scotland, and Pennsylvania.
• In the Indian Subcontinent, conglomerates form a significant portion of the Eparchean Unconformity, which is mainly visible in the Southern regions of the subcontinent.
Rounding, Sorting, and Modality
When the older mineral fragments are transported via the various mediums, the grains are subjected to erosion and weathering processes. This results in smoothing of the angular corners and jagged edges of the grains, forming the round-shaped conglomerate clasts. Also, if the minerals are transported over long distance, the degree of roundness is high, as they are acted upon continuously by high-energy currents of the eroding agents. Abrasion is the term used to denote such worn-down mineral clasts. Conglomerates with a high degree of roundness are often found to have originated in benthic and fluvial environment.
It is basically defined as the degree to which the mineral clasts are of equal size or not. Poorly sorted grains are the ones that are not of the same size. For example, the fragments that are weathered due to the action of glaciers show poorly sorted mineral clasts. These are also called diamictites. On the contrary, if the clasts are more or less of the same size, then they are referred to as well sorted. This is often seen in such rocks that originate from a lacustrine or fluvial environment.
It refers to the dominance of certain clast types in such rocks. If a rock type consists of only one type of clasts, say only quartz grains, then it is characterized by the presence of a unimodal nature. If a conglomerate shows only two main types of clasts, for example: basalt and granite, then the rock is said to show bimodal nature. If more than two types of clasts are present in the same conglomerate, then it is said to show a polymodal nature. This parameter helps us in finding out the clast origin and their parent lithology.
• Conglomerates having smaller mineral fragments can be used as dimensions stones, provided that they can be broken easily into the desired shape.
• Some varieties exhibit different colors and structures, and these can be used as semiprecious stones, or also for decorating the interior portions of homes.
• Conglomerates having very hard clast minerals are crushed and used as aggregates in road and building construction industries.
• These rocks can also be used for geological analysis of specific areas, and also for studying of prospective index rocks regarding mining of diamonds and other precious gems. The former consists of studying unconformities, collecting samples, and analyzing them in a laboratory to get the proper inferences regarding the deposition of strata and the conglomerate beds. This analysis also tells us about the depositional environment. The latter consists of exploration involving conglomerates that contain kimberlite clasts, as diamonds are usually formed in host rocks called kimberlites.
• As the mineral grains are older than the matrix, their analysis helps in answering various questions regarding the history of our planet.
• In ancient times, another use of these rocks was that they are used to make millstones for grinding wheat and barley.
In 2012, the Mars Rover Curiosity photographed conglomerate outcrops of around 10-20 cm in length, strewn across Mars. The rock clasts were more or less similar in appearance to those found on the Earth. Such rocks could not have been transported by wind action only because of the coarse-grained nature. Thus, the rounding of clasts and their typical arrangement strengthen the argument that water was present on Mars probably only few million years ago!