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Term Paper on Sedimentary Rocks
Term Paper # 1. Introduction to Sedimentary Rocks:
Sedimentary rocks are also called secondary rocks. This group includes a wide variety of rocks formed by accumulation, compaction and consolidation of sediments. The sediments may be defined as particles produced from the decay and weathering of pre-existing rocks or may be derived from remains of Dead Sea or land animals in suitable environments. The accumulation and compaction of these sediments commonly lakes place under water or at least in the presence of water.
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Availability of sediments or the solid matter making the sedimentary rocks may be from varied sources. Rocks already existing on the surface of the earth at any given point of time are exposed to the action of natural agencies surrounding or operating on them such as atmosphere, wind, water, ice and chemical environment.
The net result is breakdown of these rocks into smaller and still smaller parts, the ultimate size being of the sediments. These sediments are transported to varying distances and finally deposited in suitable basins (large depressions on the body of the earth), such as sea and ocean floors, lakes and river beds.
Some sediment may be derived as precipitates or evaporates from springs, lakes and more often from lagoons, bays and seawater. Animal and vegetable life, including microorganisms also contribute a fairly large supply of organic residues, which on gradual accumulation after the death of the source get compacted and turn into hard massive bodies of sedimentary rocks.
Sedimentary rocks are known to cover as much as 75 per cent of the surface area of the earth, the rest being covered by the igneous rocks and the metamorphic rocks. But depth wise, they disappear at shallow depths at most of the places; in fact, in the upper 16 km of the crust of the earth, the volume of sedimentary rocks is estimated to be less than ten percent, ranging between 5-8 percent in most studies.
Term Paper # 2. Formation of Sedimentary Rocks:
The process of formation of sedimentary rocks is ever prevailing. Even at present, millions of tonnes of existing rocks are broken every year in different areas of the earth by natural process. The sediments so produced are transported to the settling basins such as sea floors where they are deposited, get compacted and consolidated and finally transformed into a cohesive solid mass.
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That is a sedimentary rock. Similarly, millions of sea and oceanic organisms perish every month and their hard parts are accumulating at the floors of these water bodies. These hard parts gradually get compacted and converted into sedimentary rocks of a different origin. Some chemical processes especially evaporation and precipitation regularly operate on surface of water bodies containing dissolved salts and produce solids that settle down in those bodies.
Here is another type of sedimentary rock being formed. Similar processes are believed to have operated right from the beginning of the Earth as a distinguished planet of the Solar System. Hence, at present, formation of sedimentary rocks has to be explained with specific reference to the type of the source of the sediments/particles and the environment in which these materials have been deposited.
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Sedimentary rocks are broadly grouped into three classes on the basis of their mode of formation:
A. Mechanically formed or Clastic Rocks;
B. Organically formed Rocks and
C. Chemically formed Rocks.
The last two groups are considered as a single class and named as Non-Clastic Rocks.
A. Clastic (Mechanically Formed) Rocks:
A series of well-defined steps are involved in the formation of clastic rocks:
(a) Decay and Disintegration:
Rocks existing on the surface of the earth are exposed to decay and disintegration by the action of natural agencies like atmosphere, water and ice on them. The original hard and coherent rock bodies are gradually broken down into smaller and still smaller fragments, grains and particles. The disintegrated, loosened material so formed and accumulated near the source is called detritus. Hence, clastic rocks are often also called as detrital rocks.
(b) Transport of Sediments:
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The detritus produced from the decay and disintegration of the preexisting rocks forms the source of the sedimentary rocks but it has to be transported to a suitable place for transformation again into a rock mass. The wind, running water and ice in the form of glaciers are the very strong and common agents of transport for carrying millions of tonnes of sediments and particles from one place to another including seas and oceans.
The winds transport the sediments from ploughed fields, the deserts and dry lands in series of jumps (saltation) and in suspension modes. These loads of sediments are dropped down wherever intercepted by rains. The mightiest agents of transport of sediments are, of course, streams and rivers, all terminating into lakes or seas. The running water bodies transport the sediment load as bed-load, suspended-load and dissolved load, all dumped at the settling basins.
Ice in the form of huge moving bodies called glaciers also breaks the rocks along their bases and sides (in valley glaciers) and dumps the same at snow lines thereby making large volumes of the clastic load available for further transport by other agencies. It is easy to imagine that millions of tonnes of land mass as scratched by these surface agencies is transported to seas and oceans every year and deposited there.
(c) Gradual Deposition:
The sediments as produced through weathering and erosion are transported to settling basins. These basins may be located in different environments such as on the continents, along the seashores or in deep-sea environments. As such sedimentary rocks formed in different environments will show different inherent characters.
In the continental environments may be included the glacial deposits, the fluvial deposits, the glacio-fluvial deposits and the eolian deposits, each type giving rise to a definite type of sediment accumulation. In the marine deposits, some sediment may be dropped just along the sea-shore, or at some shallow depth within the sea or miles away in the deep-sea environment.
The most important phenomenon that happens to the sediments during their transport and deposition is SORTING or grading according to their size, shape, and density. Hence, these sediments get deposited in the form of layers in most cases. Deposition generally takes place under ordinary pressure and temperature conditions.
The sediments deposited in the settling basins gradually get converted to cohesive, hard and massive rock formations through the process of compaction, consolidation and cementation, which are collectively known as diagenesis.
(d) Diagenesis:
The process of transformation of loose sediments deposited in the settlement basins to solid cohesive rock masses either under pressure or because of cementation is collectively known as diagenesis.
It may be achieved by either of the two methods:
i. Welding
ii. Cementation.
i. Welding:
Welding is the process of compaction of the sediments accumulated in lower layers of a basin due to the pressure exerted by the load of the overlying sediments. This results in squeezing out all or most of the water from in between the sediments, thus bringing them closer and closer and consolidating them virtually in a solid rock mass. In fact the degree of packing of sediments in a sedimentary rock is broadly directly proportional to the load of the overlying sediments.
ii. Cementation:
Cementation is the process by which loose grains or sediments in a settlement basin get held together by a binding material. The binding material may be derived from within the accumulated particles or the fluids that percolate through them and also evaporate or precipitate around those particles thus binding them in a rock like mass.
B. Chemically Formed (Non-Clastic) Rocks:
Water is a great solvent. Water from rains, springs, streams, rivers, lakes and underground water bodies dissolves many compounds from the rocks with which it comes into contact. In most cases all these dissolved salts are carried by the running water to its ultimate destination – the sea.
Hence the brackish or saltish taste of the sea water. In many other cases also, the local water-bodies may get saturated with one or other dissolved salt. In all cases, a stage may be reached when the dissolved salts get crystallized out either through evaporation or through precipitation.
Thus, limestone may be formed by precipitation from carbonated water due to loss of carbon dioxide. Rock salt may be formed from sodium-chloride rich seawater merely by the process of continued evaporation in bays and lagoons. Chemically formed rocks may be thus of two types: precipitates and evaporites. Examples are limestones, rock salt, gypsum, and anhydrite.
C. Organically Formed (Non-Clastic) Rocks:
Oceans and seas cover more than 70 per cent of the globe. These extensive water bodies sustain a great variety of animal and plant life. The hard parts of many sea organisms are constituted chiefly of calcium and/or magnesium, carbonates. Death and decay of these organisms within the water bodies gradually results into huge accumulations of carbonate materials, which get compacted and consolidated in the same manner as the normal sediments.
Limestones are the best examples of organically formed sedimentary rocks. Generally the evidence of the source material gets obliterated from these rocks with the passage of time. In some cases, however, it may not be much difficult to ascertain the same from uncompacted or partly compacted fragments.
Environment of Formation:
Facies:
Sediments may accumulate at many different places in vastly different environments. The concept of formation of a sedimentary rock in a particular type of environment is explained by the term facies.
Three main facies are recognised with respect to the formation of sedimentary rocks:
(a) Continental Facies:
Sedimentary rocks formed on the continents such as in lakes, rivers, streams and alluvial fans are said to belong to the continental facies. Coarse-grained rocks like breccia, conglomerates and soft sandstone are typical examples of rocks of continental facies.
Boulder clays of glacial origin and varved clays of lacustarine origin also belong to continental facies although they differ at sub-facies level. The rocks of continental facies are, in general, relatively less dense, loosely packed, and often cemented.
(b) Transitional Facies:
Some sedimentary rocks may be formed by accumulation and compaction of sediments along the seashore, or on the continental shell that remains partly submerged under sea such as beaches and deltas. These sediments and hence rocks developed from them represent the transitional facies. Many types of sandstone, siltstone and claystone are formed in the transitional facies.
(c) Marine Facies:
All sedimentary rocks formed at sea floor and ocean floors are covered under marine facies. These may be further subdivided in shallow sea deposits and deep-marine deposits formed on ocean floors.
Term Paper # 3. Mineralogical Composition of Sedimentary Rocks:
Sedimentary rocks show great variation in their mineralogical composition. Rocks of simplest composition that is, containing one or two minerals only, (e.g. limestones) are very common. Similarly, rocks containing host of minerals like clays and shales are also not less common.
This variation is explained by following factors:
(a) The Nature of Gathering Ground:
The agents of decay, disintegration and transportation are the suppliers of the sediments. The composition of an ultimate sedimentary rock will actually reflect the sum-total of the composition of the areas over which these agents operate for obtaining the sediments.
Streams eroding carbonate-rich hills will carry mostly carbonate particles in them and, therefore, may become the source for limestone rocks; similarly those running over silica-rich terrain may contain quartz as a dominant load fraction and become the source of sandstones. But where the same stream flows over a sequence of igneous, sedimentary and metamorphic rocks, its load would be made up of different types of minerals.
(b) Duration of Transport:
This factor defines the extent in terms of time and distance for which any load of sediments is transported from the gathering ground to the settlement basin. Sediments that are soft and fragile wear out easily whereas hard, resistant and durable grains from the gathering ground are often transported to the ultimate destination. Quartz is one example of resistant minerals and hence is represented in many types of sedimentary rocks.
(c) Mixing Up of Sediments:
In rocks formed along sea shore (transitional facies) from the detritus carried up to that place by many long-distance streams, a variety of sediments may get mixed up by waves and currents before actual settling starts. Thus a rock formed in such a situation may have a complex mineralogical composition.
In the continental facies, however, the glacial environments may not allow the embedded load any chance for sorting. The morainic rocks formed at glacial terminals, therefore, may show all sorts of mineral and rock fragments occurring in the same sedimentary rock.
The bulk of most common sedimentary rocks are made up only of few common rock forming minerals like quartz (sandstones, quartzites), calcite (limestones), felspars (greywacks), gypsum and clay-minerals (shales).
(d) Allogenic and Authigenic Minerals:
Sometimes the minerals of sedimentary rocks are grouped into two classes- allogenic and authigenic. The allogenic (detrital) minerals are those which have been formed outside the basin of deposition, that is, they have been brought there by some natural agent of transport.
Quartz, felspar, amphiboles, pyroxenes, olivine and corundum are some examples of allogenic minerals. The authigenic minerals are those that have been formed within the basin of deposition. Commonly they are the result of chemical, biochemical, or biomechanical activity that takes place in the basin of deposition. Calcite, dolomite, anhydrite and gypsum are few examples of minerals of this group.
Term Paper # 4. Textures of Sedimentary Rocks:
Sedimentary rocks show considerable variation in their texture (or mutual relationship of the constituent minerals).
Their texture is determined by at least six contributing factors:
(i) Origin of Grains:
A sedimentary rock may be partially or wholly composed of clastic (or allogenic) grains, or of chemically formed or organically contributed parts. Thus the rock may show a clastic texture or a non-clastic texture.
(ii) Size of Grains:
The grain size in the sedimentary rocks varies within wide limits. Individual grains of less than 0.002 mm and more than 250 mm may form a part or whole of these rocks.
Three textures recognized on the basis of grain size are:
Coarse-grained rocks; – average grain size > 5 mm
Medium-grained rocks; – average grain size between 5 and 1 mm.
Fine-grained rocks; – average grain size < 1 mm
(iii) Shapes of Grains:
The sediments making the rocks may be of various shapes: rounded, subrounded, angular and subangular. They may show sphericity to various degrees. Roundness and sphericity are the indications of varying degree of transport and abrasion suffered during that process.
Thus, Breccias are made up mostly of rough and angular fragments indicating least transport and abrasion. Conglomerates are full of rounded and smooth-surfaced pebbles and gravels indicating lot of transport and rubbing action during their transport before getting deposited and consolidated into a rock mass.
(iv) Packing of Grains:
Sedimentary rocks may be open-packed or porous in textures or densely packed depending upon their environment of formation. The degree of packing is generally related to the load of the overlying sediments during the process of deposition.
(v) Fabric of Grains:
A given sedimentary rock may contain many elongated particles. Their orientation is studied and described in terms of orientation of their longer axes. If all or most of the elongated particles are arranged in such a way that their longer axes lie in the same general direction, the rock is said to show a high degree of preferred orientation. This direction is generally indicative of the direction of flow of the current during the period of deposition.
(vi) Crystallisation Trend:
In sedimentary rocks of chemical origin, the texture is generally defined by the degree and nature of crystallized grains. Rocks may show perfectly interlocking grains giving rise to crystalline granular texture or they may be made up of non-crystalline, colloidal particles when they are termed as amorphous.
Term Paper # 5. Structures of Sedimentary Rocks:
The term structure signifies some large-scale features developed in the rock masses during the process of their formation.
These can be studied under following three headings:
Mechanical Structure:
These are the most prevalent structures of clastic group of sedimentary rocks. They are developed due to physical processes operating at the time of deposition of the sediments.
Following are common mechanically developed structures of sedimentary rocks:
i. Stratification:
By stratification is understood a layered arrangement in a sedimentary rock. This may be developed very prominently and can be seen from a distance of miles or in other cases may have to be ascertained after close examination of the rock. The different layers, also called beds or strata may be similar or dissimilar in colour, composition, grains size and texture. Planes of weakness – the bedding planes separate the beds from each other.
The thickness of each layer in a sedimentary formation may show great variation from a few centimeters to many meters. In lateral extension, the layered structure may show continuity for several meters to hundreds of kilometers. Further, the layers may be horizontal or slightly inclined when they are undisturbed after their formation; in other cases they be steeply inclined, folded or bent and broken or overturned if affected by tectonic forces after their original deposition.
ii. Lamination:
This is also a layered structure similar to stratification as found in the sedimentary rocks. In lamination, however, the individual layers are quite thin (generally less than 1 cm. in thickness). Lamination is a characteristic structure of fine-grained sedimentary rocks like clays and shales. The individual layers are called laminae and are distinguished commonly on the basis of difference in colour.
iii. Cross Bedding:
It is a sedimentary structure in which various layers lying one above another are not parallel but bear an irregular or inclined relationship to each other. Such a structure often results from deposition having taken place in a shallow-water environment. In such environment the stream suffers repeated changes in direction of flow or the currents produced in the body of the water. The structure is sometimes referred as false bedding or current bedding.
Following are common types of false bedding:
(a) Tabular:
A type of cross bedding in which the top and bottom surfaces of the deposit are essentially parallel, indicating its deposition in the same main channel, but the intervening layers are inclined differently with respect to each other.
(b) Lenticular:
A type of cross-bedding in which all the layers show an extreme irregularity in their shape and disposition; each individual layer may be intersected by many others lying at different angles.
(c) Wedge Shaped:
In this case, the cross-bedding structure is highly complex- the individual layers exist in well-defined sets of parallel layers but these sets bear angular relationships to each other. These layered sets are sometimes mutually inclined in such a way that they give the appearance of interwoven wedges when seen in vertical cross section.
iv. Graded Bedding:
In some stratified rocks the component sediments in each layer appear to be characteristically sorted and arranged according to their grain size, the coarsest being placed at the bottom and the finest at the top. Such an individual layer is said to be graded. When a sequence of rocks is made of such graded layers, the structure is called graded bedding.
Normally such perfectly graded beds are the result of sedimentation in bodies of standing water where factor of gravitative settling from a mixed load is the predominant process. In many cases, however, the exact cause of graded bedding is far from simple and may be attributed to such unrelated processes as subaqueous landslides and submarine earthquakes.
v. Mud Cracks:
These are common structural features of many fine-grained sedimentary rocks. The structure consists of polygonal or irregular cracks spread along the surface of an exposed sedimentary layer. Their development is explained by an analogy of development of similar cracks on the surface of drying mud in shallow environments even at present. Once these cracks are covered under further layers of mud, they get preserved in the body of the deposits. They come to light once again when the overlying layers are eroded with the passage of time.
vi. Rain Prints:
These are irregular, small crater-shaped depressions seen on fine-grained dried sediments. Like mud cracks, their formation can also be explained on the analogy of present day process- rain falling forcefully on fine-grained compacted clays often makes crater like depressions. These may get dried up and subsequently preserved under another layer of mud. The imprints become a part of the deposit.
vii. Ripple Marks:
These are also quite common types of sedimentary structures of mechanical origin found in deposits made in shallow water environment. They are defined as symmetrical or asymmetrical, wave-like undulations or irregularity in a layer. Ripple marks generally result from interplay of wind action and wave action during the process of deposition.
The direction of a shallow water current can easily be effected by strong winds blowing over the current- the fine sediments get dragged along with the currents because of the waves so generated and deposited as and where the waves become weaker. Another change in the direction of the current would create another layer of deposits in opposite direction and so on.
The mud cracks, rain prints and ripple marks when encountered in sedimentary formations are taken as confirmatory evidence of the formation having been deposited in a shallow water environment.
Chemical Structures:
Many sedimentary rocks are formed due to chemical processes such as evaporation, precipitation and crystallisation. These rocks often show peculiar structures quite different from those found in clastic rocks.
A few types are described below in outline:
i. Concretionary Structures:
In this type, the sedimentary rock is made up of concretions of various shapes and dimensions. The individual concretions may be rounded, sub-rounded, rough or smooth and quite small like the egg of fish or of quite appreciable size, like that of a walnut. When large number of such concretions are cemented or compacted together, the rock is said to show a concretionary structure.
Bauxite is a typical example. The Oolitic and Pisolitic Structures are also examples of concretionary structures differentiated on the basis of size of the concretions. In the Oolitic structure, the concretions are of the size of fish eggs (0.1 to 1.0mm); the rock appears as an assemblage of fish-eggs, whereas actually it is a chemically formed sedimentary rock. In the Pisolitic structure, the individual size of a concretion is like that of a peanut. Limestones and bauxite show both these structures.
ii. Nodular Structure:
This type of structure is seen in some limestones and is differentiated by development of irregularly shaped nodules of chert, iron oxides, iron carbonates and clayey ironstones. Sometimes these nodules show elongation or flattening parallel to the bedding planes indicating their deposition away from their place of formation.
iii. Geode Structure:
A geode is actually a hollow shell of rock, the interior of which is lined with inwardly projecting crystals. Generally the rock shell is made up of chalcedony and the inner encrustations are of quartz crystals. It is believed that this type of structure resulted from crystallization of quartz crystals on inner walls of an original cavity.
Organic Structures:
Some structures develop in the sedimentary rocks due to the manner in which the organic source material gets accumulated and compacted to form a rock. The fossiliferous structure and the stromatolitic structures may be described as the organic structures.
The fossiliferous structure is due to the presence of fossils of plants or animal life in the rock. Sedimentary rocks are known as the only source of fossils. Some rocks may be highly fossiliferous whereas others may contain little or no fossils in them. The stromatolitic structure is produced by the presence of remains of algae, a kind of lower vegetation in the rocks.
Term Paper # 6. Classification of Sedimentary Rocks:
Sedimentary rocks have been variously classified on the basis of their mineralogical composition, environment of deposition, mode of formation and textural and structural features.
In this classification, all the sedimentary rocks are grouped under two main divisions:
A. The clastic and the
B. Non-clastic rocks.
This is done on the basis of their mode of formation. These groups are further subdivided on the basis of their grain size, composition and nature of the source material.
A. Clastic Rocks:
These are also called mechanically formed or detrital rocks and include all those sedimentary rocks that have been formed from pre-existing rocks by operation of four processes – weathering and erosion, transport, deposition and diagenesis.
Further classification of the clastic rocks is based on the average grain size or grade of the sediments making the rock.
Following four-fold terminology is widely followed by the sedimentalogists for grading the clastic sediments:
The gravels, sands, shales and clays are further classified to lower limits of grain size in the following manner:
(a) Gravels:
All sediments and clastic fragments of rocks above the size of 2mm irrespective of their composition and shape are broadly termed as gravels.
However, specific terms are used to name the gravels on the basis of their range of grain size in which they actually fall:
i. Boulders:
When grain size is bigger than 256 mm.
ii. Cobbles:
When grain size is between 256-16 mm
iii. Pebbles:
Grain size lies between 16-2mm
(b) Sands:
All sediments that lie within the size range of 2 mm and 1/16 mm are grouped as sands. Petrologically the term is generally used for siliceous sediments.
Sand may be further divided into three sub-groups on the basis of grain-size as follows:
i. Coarse Sands:
Size range between 2 mm and 1/2 mm
ii. Medium Sands:
Size range between 1/2 mm and 1/4 mm
iii. Fine Sands:
Size range between 1/4 and 1/16 mm.
(c) Silts:
These are very fine-sized particles of varying composition lying in the range between 1/16 mm and 1/256 mm. They are further divided into coarse, medium and fine silts although such a distinction is generally not possible with the naked eye. It is also not necessarily required for many practical applications. The silts are the major constituents of rocks known as shales.
(d) Clays:
All particles finer in size than 1/256 mm are commonly referred to as Clays. They are materials formed in a variety of ways and abound in nature both as soils and rocks as claystone, mudstones and shales, etc.
Based on the predominance of sediments of a particular grade, the clastic rocks are further subdivided into following three groups:
1. Rudites (Psephites):
These are also called rudaceous rocks and include all coarse-grained rocks of heterogeneous composition. The average grain size of the constituents in rudites is greater than 2 mm. Rudites are made up of boulders, cobbles and pebbles collectively known as gravels. These gravels are held together in the form of a rock by a natural cementing material (clay being the most common).
Examples:
Breccias and conglomerates are important examples of rudites.
2. Arenites (Psamites):
These are also called arenaceous rocks. These are made up of sediments of sand grade (2mm-1/16mm). The sand grains may be held together in the rock form either by cementation or by compaction under load from overlying sediments. In a particular rock, the sand grains of particular size range may be predominating giving rise to coarse, medium and fine arenites.
Examples:
Sandstones, grewackes and arkoses are common types of arenites.
3. Lutites (Pelites):
These are also called argillaceous rocks. Lutites may be defined as sedimentary rocks of the finest grain-size. They are made up of particles of silt and clay grades (less than 1/16mm), which are invariably packed together with varying degree of compaction. These are among the softest of rocks showing a complex behaviour towards imposed loads when found in sites of construction.
Examples:
Shales, clays, mudstones, siltstone.
Many a times a clastic rock may be made of sediments of more than one grade. It is the dominant grade that is taken into consideration while classifying the rock. If the other grade sediment is also present in a significant proportion, say more than 5 percent, it is also reflected by adopting a mixed nomenclature for the rock. For example: argillaceous sandstone, arenaceous shales, siliceous clays and so on.
B. Non-Clastic Rocks:
This group includes all those sedimentary rocks that have been formed by any one of following two processes:
(1) Operation of simple chemical processes such as evaporation, precipitation and crystallisation at ordinary temperature and pressure from natural solution in different environments;
(2) Accumulation of hard parts of organisms or remains of plant life followed by their compaction and consolidation.
The non-clastic rocks are also called non-detrital rocks. They are generally homogenous in character, fine-grained in particle size and varying in chemical composition.
This group’ is further divided into two sub-divisions:
(a) Chemically formed rocks and
(b) Organically formed rocks.
(a) Chemically Formed Rocks:
They are generally formed by precipitation, evaporation or crystallization from natural aqueous solutions carrying the weathered material in the form of dissolved load.
On the basis of their chemical composition, these rocks are further sub-divided into following groups:
(i) Siliceous Deposits:
These are the deposits in which silica (SiO2) is the chief constituent. Some forms of silica like chalcedony and opal are slightly soluble in water. When solutions saturated with this type of silica enter environments where evaporation is possible, deposits of siliceous masses are made. Examples- Flint, chert, jasper.
(ii) Carbonate Deposits:
These are precipitated from carbonate rich waters under different conditions that control the concentration of carbon dioxide. Many deposits of limestone, dolomite and magnesite are of chemical origin formed from sea waters rich in calcium carbonate and magnesium carbonate. Carbonate deposits of iron (siderite) having a similar origin are also known.
(iii) Ferruginous Deposits:
Oxides and hydroxides of iron are common examples of chemically precipitated iron deposits. At places these deposits make huge formations extending over several kilometers so that they can be extracted as a source (ore) of iron. The so-called bog- iron-ores are iron hydroxides of chemical origin.
(iv) Phosphatic Deposits:
These deposits mostly form from sea-waters rich in phosphoric acid. Some limestones and shales may also contain phosphate compounds in good proportion formed in them due to some chemical process.
(v) Evaporites:
These may be treated as a distinct class of sedimentary rocks formed by the process of evaporation. Some very important sedimentary rocks of economic value that belong to the evaporites are- rock salt anhydrite, gypsum, borates, rock sulphur and nitrate.
These deposits have formed from bodies of seawater (such as bays and estuaries) that got detached from the main sea. Loss of moisture from these bodies due to evaporation with passage of time increased the concentration of the salts to an extent that these salts separated out as rock masses. Even at present common salt is manufactured from seawater by the same process.
(b) Organic Deposits:
Sedimentary deposits formed exclusively or predominantly from remains of organisms (both plants and animals) fall under this group. The organisms might have contributed in the formation of these deposits directly or indirectly.
The rocks in which the bulk of their material is made of the remains of the organisms have their direct contribution. Coral limestone is an example. Indirect contribution is made by organism in a different manner. Some types of bacteria may help or even be solely responsible for precipitating the rock components from solution.
Following types of organic deposits are distinguished on the basis of their chemical composition:
(i) Carbonate Rocks:
A great part of the limestones found in different areas of the world is actually marine and organic in origin. It has been formed by gradual accumulation and compaction of shells and skeletal bones of sea organisms like foraminifera, corals, crinoids and crustacea etc.
(ii) Carbonaceous Rocks:
Sedimentary rocks rich in carbon are called as carbonaceous rocks. In their formation, the source material for carbon is mainly derived from plants. Some carbonaceous shales may be cited as examples. Coals are also carbonaceous materials in their first stage of formation. Wood gets accumulated in huge volumes in sedimentary basins. Biomechanical and bio-chemical processes convert the wood to various grades of coal.
(iii) Phosphatic Deposits-Guano:
Most phosphate rocks are of chemical origin. Guano is the name given to small volumes of phosphatic composition that are actually accumulations of excreta of some birds and hence considered organic in origin. These birds live on islands and mostly eat fish. Their excreta naturally contain high content of phosphate. When accumulated over a period of time guano forms rich source of phosphate salt. Large deposits of guano occur in islands of eastern Pacific Ocean and also in West Coast of India.
(iv) Ferruginous Deposits:
These are mostly iron carbonate deposits of good volume. In many fresh water lakes and also in swamps some bacteria are thought to be responsible for reduction of ferric oxide to ferrous oxide and finally to its precipitation as iron carbonate. Hence the organic origin of some siderite that is used as ore of iron.
Miscellaneous Deposits:
Some sedimentary rocks have complex mode of formation so that they cannot be easily placed under any one of the above classes. It is customary to describe them separately. These include Bauxite, Terra Rosa and Laterites. Weathering, (decay and decomposition) of pre-existing rocks accompanied with many other chemical processes are believed to have cooperated intimately in their formation.
Term Paper # 7. Engineering Importance of Sedimentary Rocks:
Sedimentary rocks cover a great part of the crust of the earth; they make up more that 75 percent of the surface area of the land mass. It is with these types of rocks that an engineer has to deal with in majority of cases. It is, therefore, essential for a civil engineer to know as much as is possible about the salient features of these rocks.
He has to see, for instance, if such rocks would withstand loads under heavy construction and also, if they could be trusted in cuts and tunnels in highway construction and also as reservoirs. They are the most important rocks to act as natural reservoirs of oil and ground water supplies.