Here is a list of twelve important sedimentary rocks: 1. Breccia 2. Conglomerates 3. Sandstones 4. Shale 5. Limestones 6. Dolomite 7. Coals 8. Iron Ores 9. Gypsum 11. Rock Salt 12. Flint and Chert.
It is a mechanically formed sedimentary rock classed as Rudite. It consists of angular fragments of heterogeneous composition embedded in a fine matrix of clayey material. The fragments making breccia are greater than 2 mm average diameter but sometimes these may be quite big in dimensions. The angularity of the fragments indicates that these have suffered very little or even no transport after their disintegration from the parent rocks.
On the basis of their source, following types of breccia are commonly recognized:
(a) Basal Breccia:
This rock is formed by the sea waters advancing over a coastal region covered with fragments of chert and other similar rocks. The advancing waters supply the fine mud, which is spread over the rock fragments and acts as a binding material. Once the seawater retreats, the loose chert fragments get cemented together as breccia rocks.
(b) Fault Breccia:
This rock is also called crush-breccia. Such rocks are so named because they are made up of angular fragments that have been produced during the process of faulting. The fragments so produced due to crushing effect of the block movements subsequently get embedded in clay and other fine material (often also derived during the faulting process and called gouge) and ultimately form a cemented rock – the crush breccia.
(c) Agglomeratic Breccia:
It is a specific type of breccia containing angular and sub-angular fragments derived from volcanic eruptions. It may also contain some fused material that has been cemented together with the solid material broken and thrown out of the craters.
These are sedimentary rocks of clastic nature and also belong to rudaceous group. They consist mostly of rounded fragments of various sizes but generally above 2 mm, cemented together in clayey or ferruginous or mixed matrix. The roundness of gravels making the rock is a useful characteristic to differentiate it from breccia in which the fragments are essentially angular. The roundness indicates that the constituent gravels have been transported to considerable distances before their deposition and transformation into conglomerate rock.
The constituent gravels of conglomerates may differ considerably in their chemical composition- they may be pure minerals or rock gravels of different classes that got cemented together in a natural manner. Similarly, the cementing matrix may be siliceous, calcareous or of mixed type.
Conglomerates are generally distinguished on the basis of the dominant grade of the constituent gravels in following three types:
Boulder-Conglomerates (gravels > 256mm)
Cobble-Conglomerate (gravels: 64—256 mm)
Pebble-Conglomerate (gravels: 2—64 mm)
Sometimes the conglomerates are distinguished on the basis of source of the gravels, as:
Having gravels derived from advancing sea-waves over subsiding land masses;
In which gravel making the conglomerates are distinctly of glacial origin;
In which gravels are of distinct volcanic origin but have subsequently been subjected to lot of transport resulting in their smoothening and polishing by river transport before their deposition and compaction or cementation.
On lithological basis (that is, the type of rock they are made up of), conglomerates are grouped in two classes:
Simple in composition, these gravels are made up of quartz, chert and calcite;
In these conglomerates the constituent gravels are derived from rocks of all sorts- igneous, sedimentary and metamorphic, all cemented together. The so-called Fanglomerates are conglomerates formed and found at the base of alluvial fans and cones.
Conglomerates are of special geological significance when they occur in the form of well-defined layers of good thickness in sedimentary formations. They are indicative of shallow- water phase in the depositional environment.
Sandstones are mechanically formed sedimentary rocks of Arenaceous Group. These are mostly composed of sand grade particles that have been compacted and consolidated together in the form of beds in basins of sedimentation. The component grains of sandstones generally range in size between 2 mm and 1/16 mm. Silica in the form of very resistant mineral QUARTZ is the dominant mineral constituent of most sandstones.
Quartz (SiO2) is the most common mineral making the sandstones. In fact some varieties of sandstone are made up entirely of quartz. Besides quartz, minerals like felspars, micas, garnet and magnetite may also be found in small proportions in many sandstones.
In some sandstone the component grains may be cemented together by a cementing material that may be siliceous, calcareous, argillaceous or ferruginous in composition. In other sandstones, the component minerals may be welded together by natural pressures from overlying sediments.
Sandstones are, in general, medium to fine-grained in texture. The component grains show a great variation in their size, shape and arrangement in different varieties.
Thus, when the texture is determined on the basis of the grade of the component grains, three types are recognized:
The individual grains may be round or angular in outline, loosely packed or densely packed and in simple or interlocking arrangement. The shape and mutual arrangement of the component grains, or the texture, is greatly responsible for the engineering and other properties of sandstone. In fact, the properties of porosity and permeability of these rocks are the critical parameters that make them useful or useless in different situations.
Sandstones naturally occur in a variety of colours- red, brown, grey and white being the most common colours. The colour of sandstone depends on its composition, especially nature of the cementing material. For example, presence of iron oxide is responsible for the red, brown and yellow shades; presence of glauconite gives a greenish shade to the sandstones.
Many types of sandstones are distinguished on the basis of their composition and the nature of the cementing material.
Following types are of common occurrence:
(i) Siliceous Sandstones:
Silica (SiO2) is the cementing material in these sandstones. Sometimes the quality of the siliceous cement is so dense and uniform that a massive compact and homogeneous rock is formed. This is named QUARTZITE. This type of sedimentary quartzite, when subjected to loading fractures across the grains showing clearly very dense nature and homogeneity of the cementing silica with the main constituent silica of the rock.
(ii) Calcareous Sandstones:
Calcareous Sandstones are those varieties of sandstones in which carbonates of calcium and magnesium are the cementing materials.
(iii) Argillaceous Sandstones:
These are among the soft varieties of sandstone because the cementing material is clay that has not much inherent strength.
(iv) Ferruginous Sandstones:
As the name indicates, the cementing material is an iron oxide compound.
On the basis of mineralogical composition, following types of sandstones are commonly recognized:
This is a variety of sandstone that is exceptionally rich in felspar minerals besides the main constituent quartz. It is believed that these rocks are formed due to relatively quick deposition of detritus derived from weathering and disintegration of crystalline igneous and metamorphic rocks like granites and gneisses respectively. Arkose rock generally occurs in horizons that can be genetically related to some crystalline massif occurring in close neighbourhood.
These are broadly defined as grey coloured sandstones having a complex mineralogical composition. They contain a fine-grained matrix. In this matrix, grains of quartz and some felspars are found embedded side by side with fragments of rocks like felsites, granites, shales etc.
The exact composition of the matrix is so complex that it may not be easily determined in most cases. However, it is invariably clayey in nature, interspersed with particles of pyrite, epidote, mica, quartz and felspars. The minerals and rock-constituents are generally angular to subangular in outlines indicating least transport before compaction.
It is a variety of sandstone that is exceptionally rich in mica dispersed in parallel or subparallel layers. The abundance as well as arrangement of mica, typically muscovite, renders the stone weak and easily splitting. Hence its use in load bearing situations is not recommended.
It is a massive variety of sandstone that is rich in quartz and does not contain bedding planes or any mica. It is compact, dense, massive and a strong rock suitable for construction demanding high crushing strength.
It is another type of sandstone consisting of angular and sub-angular quartz grains and cement of secondary quartz with some kaolin.
Sandstones of hard, massive and compact character are very useful natural resources. They are most commonly used as materials of construction: building stones, pavement stones, road stones and also as a source material for concrete. The Red Fort of India is made up of red sandstones.
Next to shales, sandstones are the most abundant sedimentary rocks found in the upper 15 km of the crust and make an estimated 15 percent of total sedimentary rocks of the earth.
Shale is a fine-grained sedimentary rock of argillaceous (clayey) composition. Shales are generally characterized with a distinct fissility (parting) parallel to the bedding planes and are made up of very fine particles of silt grade and to some extent of clay. Besides fissility, some shales show the laminated structure.
The exact mineralogical composition of shales is often difficult to ascertain because of the very fine size of the constituents. Generally speaking, shales are very intimate mixtures of quartz, clay minerals and accessory minerals like oxides of iron, carbonates, and organic matter. Silica and clay minerals together make more than seventy percent of shales in most cases.
Chemically speaking, shales exhibit still greater variation.
According to F.W. Clarke, the average chemical composition of typical SHALE rock is as follows:
Shales are characterized with a distinct property of fissility, which may be defined as “tendency of a rock to split into flat, shell-like fragments parallel to bedding.”
The fissility of shales is partly primary and partly secondary in nature. The primary fissility results during the process of deposition itself when the mica-like constituents of shale tend to get deposited in parallel orientation. The secondary fissility develops after the deposition when under the imposed load of layers of sediments; the flaky and platy minerals present in the lower layers of shale (having a primary fissility) undergo further orientation imparting the ultimate fissile character to the rock.
Shales also show lamination or deposition in very thin layers. The laminae or the layers may range in thickness from 0.05 mm to 1.00 mm depending upon the environment of deposition.
Shales have been classified variously.
For example, F.J. Pettijohn divides them into following three classes on the basis of their origin:
(i) Residual Shales:
These are formed from decay and decomposition of pre-existing rocks followed by compaction and consolidation of the particles in adjoining basins without much mixing;
(ii) Transported Shales:
These are deposits of clastic materials of finer dimensions transported over wide distances before final settlement in basins of deposition.
(iii) Hybrid Shales:
In such shales, materials derived both from clastic sources and non-clastic especially those from organic sources make up the rock.
In another classification, which is due to W.C. Krumbein and L.L. Sloss, four types of shales are recognized on the basis of their mineralogical composition:
(i) Quartz Shales:
These are rich in free quartz content.
(ii) Felspathic Shales:
The shales in which felspars and clay minerals predominate; silica becomes a secondary constituent.
(iii) Chloride Shales:
In these shales, minerals of chlorite group and clay-group make the bulk of the shales.
(iv) Micaceous Shales:
These are rich in muscovite mica and other flaky and play minerals.
In a simpler classification, shales are distinguished into four classes on the basis of the predominant group of sediments- Siliceous shales, Calcareous shales. Ferruginous shales, and carbonaceous shales. OIL SHALES are a type of carbonaceous shales that contain rich amount of organic matter in fairly decomposed form. The so-called black shales may contain about 25 percent or more of organic matter.
Shales are formed from compaction and consolidation of sediments of silt and clay grades materials. The process starts from the compaction of mud rich in moisture. Compaction of mud, generally due to load from overlying sediments, results in squeezing out of water resulting in its shrinkage. Compacted muds that still retain 10-15% of moisture are lithologically (and not mineralogically) termed as clays.
However, continued compaction may result in further loss of moisture and during this stage, there may take place orientation of the clay and platy minerals in parallel or sub-parallel layers. If orientation (within the particles) does take place, the deposit attains the property of fissility, it becomes shale. If it gets compacted further and without any fissility, it may simply be a mudstone or claystone and not a shale.
As regards the ideal environment of formation of shales, a marine environment, with a quieter water and depth of 50 meters or more, is most suitable for the deposition of clays and their gradual transformation into shales at the lower levels. The fine load is transported to these places by streams and even by winds.
Shales are variously used for manufacture of bricks and tiles. These are at place source of alumina, paraffin and oil.
Of all the sedimentary rocks occurring on the surface of the earth, shales are the most predominant forming 70-80 percent of this group. These rocks occur in massive formations and beds extending over several hundred kilometers in many cases.
These are the most common sedimentary rocks from the non-clastic-group and are composed chiefly of carbonate of calcium with subordinate proportions of carbonate of magnesium. They are formed both bio-chemically and mechanically.
Pure limestone is invariably made up of mineral calcite (CaCO3). In the limestone rock formations, however, presence of dolomite CaMg (CO3)2, quartz (SiO2), felspar minerals and iron oxides is rather a common feature.
In terms of chemical composition, limestones are chiefly made up of CaO and CO2. Magnesium Oxide is a common impurity in most limestones; in some its percentage may exceed 2 percent, the rock is then called magnesian limestone. Other oxides that may be present in limestone are- silicon dioxide, ferrous and ferric oxides (or carbonates); and aluminium oxide. Strontium oxide is also present in some limestones as a trace element.
In view of the diverse ways in which the limestones are formed, these rocks show a great variety of textures. The most important textural feature of limestones is their fossiliferous nature. Fossils in all stages of preservation may be found occurring in limestones. Other varieties of limestones show dense and compact texture; some may be loosely packed and highly porous; others may be compact and homogeneous. Concretionary texture is also common in limestones.
Many varieties of limestones are known. Broadly speaking these can be divided into two groups- autochthonous and allochthonous. The first group includes those varieties which have been formed by biogenic precipitation from seawaters. The allochthonous types are formed from the precipitated calcareous sediments that have been transported from one place to another where they were finally deposited.
Following are common types of limestones:
It is the purest form of limestone characterised by fine-grained earthy texture. Common colour of chalk is white. Some chalks may be exceptionally rich in the remains of very small sea organisms called foraminifera.
b. Shelly Limestone:
Also called fossiliferous limestone, it has a rich assemblage of fossils that are fully or partly preserved. When the limestone is made up entirely of fossils, it is termed coquina.
c. Argillaceous Limestone:
These limestones contain clay as a significant constituent and are clearly of allochthonous origin. When the clay and carbonate factions are present in almost equal proportions, the rock is termed marl.
d. Lithographic Limestones:
These are compact massive homogeneous varieties of pure limestones that find extensive use in litho-printing.
It is a common nodular or concretionary form of carbonate material formed by evaporation of subsoil water rich in calcium carbonate just near the soil surface. It is non- marine in origin.
It is a carbonate deposit formed by precipitation from carbonate rich spring waters. These deposits are also known as travertine or calc-tuffa and commonly occur around margins of Hot Springs.
Of all the varieties of limestone, the commonest is the normal marine limestone.
As has been said above, limestone may be purely organic or inorganic in origin.
The three different environments of formation of limestones are:
(i) Biohermal Limestones:
Which occur in the form of reefs or mounds and are actually transformed deposits of corals and similar sea organisms? These are highly fossiliferous.
(ii) Biostromal Limestones:
These are sheet-like accumulations of biogenic deposits that may have single or complex types of organisms involved in their formation. These may represent accumulations of limestone precipitated from local solutions or precipitates from other lime- secreting areas.
(iii) Pelagic Limestones:
These are formed from the accumulations of limy secretion of floating type of sea organisms, such as foraminifera. These are mostly free from fossils of bigger types of sea-organisms.
The mechanically deposited limestones are formed by accumulation of particle of calcite derived from pre-existing rocks in much the same way as those of any other clastic rock.
Limestones and dolomites find important applications in many industries and engineering practice. Thus, limestone is a primary source material for manufacture of Portland cement and for a large variety of limes. Its other uses include those in metallurgical industries as a flux and in construction practice as building stone and road stone. Limestone is also used in chemical industries. Pure dolomite is a good source of magnesium.
Limestones and dolomites are among the most common non-clastic sedimentary rocks forming mountains and hills extending over several hundred kilometers at a stretch in many regions of the world.
It is a carbonate rock of sedimentary origin and is made up chiefly – more than 50 percent – of the mineral dolomite which is a double carbonate of calcium and magnesium with a formula of CaMg(CO3)2. Ferrous iron is present in small proportions in some varieties. Gypsum also makes appearance in some dolomites. But the chief associated carbonate is that of calcium, in the form of calcite.
Dolomite shows textures mostly similar to limestones to which it is very often genetically related. In other varieties, dolomites may be coarsely crystalline, finely crystalline or showing interlocking crystals. In the coarsely crystalline varieties, the mineral dolomite has a typical rhombohedral habit.
Dolomites are formed in most cases from limestones by a simple process of replacement of Ca++ ions by Mg++ ions through the action of Mg++ ion rich waters. This ionic replacement process is often termed dolomitization. The replacement may have started shortly after the deposition of limestone or quite subsequent to their compaction.
Direct precipitation of dolomites from magnesium rich waters is also possible. Such directly precipitated deposits of magnesium carbonate occur in association with gypsum, anhydrite and calcite. It is believed that in such cases, it is the calcite, which is precipitated first, depleting the seawater of CaCO3 and enriching it with MgCO3. The CaMg (CO3)2 precipitates at a later stage.
Dolomitization by replacement method, however, is believed to be the most common method of formation of dolomites.
Dolomite is a widespread sedimentary rock and is found commonly associated with limestones. It forms intervening layers between limestone formations spread over wide areas. Also, it may occur at the extended boundaries of many limestone deposits. These indicate locations where magnesium rich ground waters could have an easy access for the replacement process to take place in an original limestone rock.
Dolomite is so closely related to limestone in composition, texture, structure and physical properties that it may not always be easily possible to differentiate between the two rocks in hand specimens.
Following characters may prove helpful:
It is obvious that the above distinguishing characteristics can in no case be considered reliable. The conclusive test shall always be a chemical analysis of the rock that should prove predominance of CaMg (CO3)2 over CaCO3 to declare it a dolomite.
These may broadly be defined as metamorphosed sedimentary rocks of carbonaceous character in which the raw material has mostly been supplied by plants of various groups. The original raw material passes through many biomechanical and biochemical processes before it becomes a coal in technical terms.
In most cases coals represent carbonized wood. The process of coal formation involves a series of stages similar to formation of sedimentary rocks such as wastage of forests and transport of the wood material through different natural agencies to places of deposition, accumulation of the material in huge formations, its burial under clays and other matter and its compaction and consolidation under superimposed load.
Biochemical transformation of the organic matter so accumulated starts and is completed under the influence of aerobic and anaerobic bacteria available at the place of deposition. The degree of carbonification depends to a great extent on the time and type of environment in which the above processes have operated on the source material giving rise to different varieties of coal.
Coals are generally classified on the basis of their carbon content (termed as fixed carbon). The fixed carbon also determines to a great extent their calorific value.
Following are main varieties of coal:
It is the lowest grade coal that consists of only slightly altered vegetable matter. It may not be even considered as a coal. It has very low calorific value, high percentage of moisture and is rich in volatile matter.
It is also known as brown coal and forms the poorest grade of coal with calorific value ranging between 6300-8300 B.th.U. It is compact and massive in structure with an upper specific gravity of 1.5 and hardness of 2.5 on Mohs’ Scale of Hardness. Some varieties of lignite may still show to a good extent the traces of original vegetable structure.
(iii) Bituminous Coals:
These form a broad group of common coals having essential properties varying within wide limits. The fixed carbon ranges between 69-78 per cent and the calorific value between 9,500 B.th.U to 14,000 B.th.U. Their common character is that they contain enough volatile matter, which makes them quite soft on heating, and they start agglomerating. Some of bituminous coals may contain volatile matter to such a high extent as 30 per cent of their bulk. Some such coals are typically banded in structure.
It is considered the highest-grade coal with fixed carbon ranging between 92- 98 per cent. It has highest calorific value in coals and burns almost without any smoke, as the volatile matter is negligible.
Coals of different varieties are found to occur almost in all countries of the world, though in varying proportions. Coals form all-purpose fuels, some varieties being more suitable for specific industrial uses.
8. Iron Ores of Sedimentary Origin:
Most of the iron ores of the world are of sedimentary origin. These iron ores form beds or layers of variable thickness that occur interstratified with other sedimentary rocks. Sedimentary iron deposits are regarded having formed chiefly as chemical precipitates in the form of oxides, carbonates and silicates from marine waters rich in corresponding salts.
Metasomatic replacement has also been suggested as another important process for formation of many iron ore deposits. It is also suggested that certain type of bacteria play considerable role in the precipitation of iron.
Indian example of iron-ore deposits of sedimentary origin is provided by the well-known Iron- Ore Series of Singhbhum, Orissa. These deposits, which are believed to be of upper Dharwarian age, occur interbedded with other rocks like phyllites and are made up of oxides, chiefly hematite, Fe2O3. These are spread in the form of a belt developed over more than 100 km.
It is a sedimentary rock composed of the mineral of the same name-gypsum, which has a composition of CaSO4.2H2O. Its common colour is white but it may also occur in other shades such as yellow, red or dark grey due to impurities present in the rock.
Gypsum is formed in nature as a result of evaporation from sea-waters rich in sulphate salts. In many cases gypsum occurs associated with rock salt bodies although independent deposits of gypsum are also quite common.
ANHYDRITE is a granular aggregate of mineral anhydrite, CaSO4, and is genetically related to the mineral gypsum- hydration of anhydrite results in gypsum. These rocks are commonly associated in occurrence.
Gypsum finds extensive uses in many industries, e.g.:
(i) As a raw material in the manufacture of fertilizers;
(ii) As an essential ingredient in the manufacture of Cement;
(iii) In the manufacture of Plaster of Paris.
(iv) As fire proofing component of gypsum boards.
10. Rock Salt:
It is also a sedimentary rock composed of mineral halite (NaCl). The texture of rock salt varies from coarse-grained crystalline to fine-grained massive. The purest rock salt is white in colour but it may occur in various other shades as grayish and reddish due to presence of impurities.
Rock salt occurs in many parts of the world interbedded with other sedimentary formations. It is commonly associated with other evaporites. It is believed to have been formed by evaporation of concentrated saline sea-water. Some layers of rock salt are exceptionally thick—100 m or more. Their formation is not easily explained by simple process of evaporation. Subsidence of the basin of deposition during the process of evaporation has been suggested by some as a possible explanation.
11. Flint and Chert:
Flint is a dark coloured sedimentary rock of siliceous composition consisting chiefly of chalcedony and extremely fine-grained quartz. It occurs commonly as concretions or nodules in chalk (limestone) deposits.
Chert is also a sedimentary rock composed of cryptocrystalline silica showing great variety of colours. It is more common in occurrence compared to flint and occurs in the form of beds or layers within limestones and other deposits.
In most cases, flint and chert are closely associated in properties and may be even taken as one rock.
Their origin may be due to any of following two causes:
(a) Primary Precipitation:
It is believed that under special environments chert gets precipitated inorganically from seawater rich in amorphous silica. The theory is yet considered inadequate because modern seawaters are generally quite under-saturated with amorphous silica.
Waters containing amorphous silica from siliceous skeletal sources are thought to have replaced limestones forming concretions and nodules of flint by the process of replacement.
It is a sedimentary rock of glacial origin. It is characterised by a structureless matrix that has fragments of various sizes, shapes and composition embedded in it. Most of these embedded fragments bear striations and other evidence of their transport by glaciers before their deposition and compaction.
The name is derived from the fact that the rock is merely a compacted and consolidated form of the glacial debris called till. The matrix or ground mass of the till is generally of grey to greenish appearance whereas the embedded fragments are of extremely heterogeneous character.