Sandstones Classification

Sandstone is a type of rocks which are neither too rough nor too fine and whose origin can be any type of rocks namely Igneous or metamorphic but someone asked you if sandstone is an example of which type of rock then the answer is sedimentary rocks. The classification of the sandstone can be done basically on two important factors i.e. texture as well as mineral composition. The two famous schemes of classifications are given by Robert H. Dott (1964) who was an American Petrologist and whose scheme was based on the concepts of P.D. Krynine & F.J. Pettijohn whereas another scheme was given by R.L. Folk (1974). The external structure because of the depositional activities as well as the composition of the sandstones is a very important factor to understand these rocks. In this article, we will learn what sandstones are and the types of sandstone in detail which will be helpful to understand this topic in Geography, Geology, Petrology, etc.

Introduction

If you are asked, sandstone is an example of which type of rock; then the answer will be those rocks whose large distribution is found on the Earth i.e. sedimentary rocks. The layers of sandstone layers can be found in each part and around 20 - 25% of these layers are found. It is composed of sediment materials that are formed from other processes such as weathering & erosion of other rocks. The rough materials will be smaller whereas smoother materials are those which depend on how far those materials are being transported. The size rate of sandstones lies between 0.0625mm ( 1/16 ) until 2 mm. The size of the material, which is called sand that can be very rough until the smallest material is very fine. These have a moderate grain size, neither too rough nor too fine. Therefore, it is easier to form the grain size of this sand in order to transport & to make them deposited. This is the reason sand deposits or sandstone can be found almost everywhere. We can know more about sandstone by its classification and when we know about quartz, feldspar, & rock fragments that are present in the sandstone, names of the sandstones can be known by their classification as well.  

Types of Sandstone

The sandstone classification scheme of Dott (1964) is one of the various classification schemes which is used by Geologists for the classification of sandstones. His scheme is considered as a modification of Gilbert's classification of silicate sandstones whereas it also incorporates dual textural & compositional maturity concepts of R.L. Folk in only one classification system. This is based on the mineralogy of framework grains as well as on the type of matrix present in between them. In this, he has set the boundary at a 15% matrix between arenite & wackes. Besides these, he also breaks up various types of framework grains into 3 categories that can be found in the sandstone i.e quartz, feldspar as well as Lithic grains.  The types are mentioned below in detail:

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Arenites

Arenites are a type of sedimentary rocks or basically, we can say a type of sandstone whose grain size lies between 0.0625 mm and 2 mm as well as it has also less than 15% clay matrix.

• Quartz Arenites

These arenites are those that contain more than 90% detrital quartz and these can also include small amounts of other grains such as feldspar, lithic fragments, etc. along with resistant grains like chert & minerals in the ZTR index. Quartz sandstone is simply rich in quartz content along with other material. Extensive weathering that occurs during or before the transport removed everything but quartz grains are more stable which results in this sandstone. Quartz Arenites are also known as ortho quartzites and sometimes also referred to as quartzose sandstone as well. These arenites are considered as the most possible & mature sedimentary rocks which are often known as ultra/super mature along with both textural as well as compositional maturity & are generally cemented by silica. The two primary environments ( usually sedimentary depositional areas ) that produce these types of sandstones are beaches or upper shoreface as well as aeolian processes. Mostly these sediments are being eroded and reworked over & over which leads to the formation of a new kind of sediments as well as rocks which are generally referred to as multicycle sand.

• Feldspathic Arenites

These arenites are those sandstones that contain less than 90% of quartz but more of feldspar. Here, feldspar is the main component & contains other unstable components as well such as lithic fragments & other minerals such as mica and heavy minerals, etc. These types of sandstones are having a pink/red colour basically due to the potassium feldspar or iron oxide as well as light grey to white and medium to coarse-grained minerals. If we talk about its texture maturity, it is usually immature whereas in terms of sorting of grains it is moderate to poorly sorted. These arenites can be judged from the sediments structure and generally occur on the craton or the continental shelf as well as sedimentary basins. The minerals contained in these type of sandstones are mostly associated with the form of plagioclase feldspar which is generally derived from quartz diorite as well as volcanic rock. The feldspar in this sandstone associated with arid to cold climates when the process of weathering is reduced.

• Lithic Arenites

These are the arenites that consist of unstable lithic fragments such as volcanic & metamorphic rocks, and also includes chert etc. These may consist of less than 90% quartz grain whereas rock fragments are considered to be more unstable than feldspar. If we talk about the colour of these sandstones they are usually light grey & dark grey whereas if we talk about sorting of grains, that are poorly sorted and textural maturity is immature to sub mature. They are generally formed under the environment where unstable material is found and may occur on marine turbidite sediments & fluvial and are generally found on the alluvial fan but can also be found in the Foreland basin in the fold-thrust belt.

Wackes Sandstone

Wackes are that sandstone that consists of more than 15% of the fine-grained clay matrix. It is also known as dirty sandstone which is a sedimentary rock with sand-sized grains of 0.063mm to 2 mm which is generally composed of rock fragments of a wide range of minerals such as pyroxenes, amphiboles, feldspars, and quartz, etc. and generally are poor sorted and angular grains. Here, the matrix has appreciable amounts of clay minerals which may constitute up to 50 %  of the volume and of the clay minerals, chlorite & biotite are more abundant as compared to muscovite & illite whereas kaolinite is absent. Thus the abundant matrix leads to binding the grains strongly which results in forming relatively hard rock. Here, quartz wackes are uncommon whereas feldspathic wackes are those type of sandstones that contain a greater than 15% matrix and Lithic wacke is a kind of sandstone which consists of greater than 15% the matrix.

Arkose Sandstone

Arkose sandstones are those sandstones that consists of at least or more than 25% of feldspar. It is rich in feldspar along with quartz as a dominant feature and also consists of mica or rock fragments as well. Sometimes it also consists of small amounts of calcite cement too. The grains are poorly rounded as well as less sorted as compared to pure quartz sandstone rock. If we talk about its colour, it is grey to reddish. The grains are usually fine to very coarse but end towards the coarser side in the scale. If we talk about fossils in these type of sandstones, they are considered rare. They are formed from the weathering of igneous or metamorphic rocks which are feldspar-rich as granitic rocks are most common, that is primarily composed of quartz & feldspar. These sediments need to be deposited rapidly in a cold/arid environment in such a way that feldspar does not undergo a significant process of chemical weathering as well as decomposition which is why this sandstone is considered texturally immature. 

Greywacke Sandstone

These types of sandstones are a heterogeneous mixture of lithic fragments as well as angular grains of quartz and feldspar & others. These are those sandstones that are characterized by quartz, feldspar, as well as small rock or lithic fragments which are generally poorly sorted angular grains & grey colour stone. It is a type of sedimentary rock that is usually found in Paleozoic strata and is texturally immature. They usually are grey, brown, yellow and black, dull-coloured rocks. These sandstones can contain a variety of minerals such as quartz, orthoclase & plagioclase feldspars, calcite, iron oxides & graphitic, carbonaceous matters, along with fragments of felsite, chert, slate, gneiss, various schists, & quartzite. These also can contain minerals such as biotite, chlorite, epidote, garnet, hornblende, tourmaline, augite, apatite,  sphene & pyrites. They are usually along with the edges of continental shelves or bottoms of trenches in the ocean as well as bases of mountain formational areas.  They can also be found along with shales as well as limestones and do not contain fossils but organic remains are common to be seen in the finer beds which are associated with them. 

Folk's Classification

Robert L. Folk who was an American Petrologist as well as professor of the University of Texas gave a descriptive type of classification of sedimentary rocks. We will learn about Folk's sandstone classification here. The basic philosophy of his says that the name of the rock must be able to convey as much information that is possible without knowing the complete description. With this respect, he proposed 5 vital sandstones properties use as defining characteristics and these properties are given below:

1. Grain size

2. Chemically Precipitated Cement, 

3. Textural Maturity

4. Miscellaneous transported constituents 

5. Clan designation

Folk said that in these properties, 2 and 4 are optional categories because they are not always observed in all type of sandstones whereas the other 3 are important ones and are always observed. The examples of rock names are mentioned below by using the Folk's fivefold names:

He also used composition to classify the sandstones as similar to other schemes given before him on the basis of quartz (Q), feldspars (F), & rock fragments ( R) and the abundance of these components leads to help in knowing and classifying the sandstones and also on the basis of these, names are also being recognised. To understand and define the name of the clan, the sum of these components should be equal to 100% and those which do not fit are being disregarded. This QFR diagram helps a lot which is given below:

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Another Classification

On the basis of hardness as well as colour, sandstones can be classified into four categories i.e grey sandstone, crystallized sandstone, hard sandstone & carbonate cemented sandstone.

• Gray Sandstones are those sandstones that consist of a mixture of the dark as well as light grains that lead to a rock that is grey in appearance and also which is generally not as hard as other types of sandstone. The hardness of this sandstone can be determined by breaking the core as well as by examining the broken core's edges. The edge of this kind of sandstone core is relatively weak & crumbles with minor impact. Cores of other types of sandstone fracture with sharp edges and shatter into small as well as sharp fragments. 

• Crystallized Sandstones are those sandstones that are white as well as sugary and very hard & brittle in many cases. In some of the sandstones of this type, the grains can be seen as welded together as well as the cored rock has a smooth and polished appearance.

• Hard Sandstones are those sandstones whose characteristics lie in-between grey sandstones as well as crystallized sandstones. These are the sandstones that are light to dark grey in colour similar to grey sandstones and also hard & brittle similar to crystallized sandstones. Some hard sandstones examples are specimens 24L, 24R, & upper 28R (this is the broken butt end of the core). This butt end can be compared with lower 28L which is considered as a mixture of light grey, dark grey as well as tan grains whereas should also be compared to the pure white grains in the specimen of crystallized sandstone such as upper 28L.

• Carbonate Cemented Sandstones are considered to be relatively rare sandstones & generally they are grey in colour and these types of sandstones grains are held together with calcium or iron carbonate cement. This kind of cement make the hardcore & often create a brown or yellow stain on the core sides such as 26R. When 10% HCL i.e hydrochloric acid is applied to the rock then calcium carbonate cement can be detected by effervescence whereas if we talk about Iron carbonates, they do not effervesce but they do have a deep brown as well as a sugary appearance on the broken butt end of the core such as e.g. lower 28R.

Additional Information 

Sandstones can be simply classified on the basis of their colours. The images of different types of sandstones are given below:

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Conclusion

Thus, to sum up in the end we can say that on the basis of composition as well as structure of the rocks, we can learn about the sandstones and types of sandstone. The major components include quartz, feldspar as well as Lithic fragments. These sandstones are formed of sediment material that can originate from any kind of rocks i.e. igneous rocks or metamorphic rocks. In this article, we learn about sandstone classification given by Dott in which we learned Arenites sandstone, Wackes Sandstone, Arkose Sandstone and Greywacke Sandstone in which we learned different mineral composition and about their texture and other aspects. This topic will be helpful in Geography, Geology, Petrology, etc. to understand sandstones.

We have learned about various types of sandstone through different classifications. Let's have a look at important questions related to it: