Explain Sedimentary Rocks in Brief
Explain Sedimentary Rocks in Brief
Sedimentary rocks are created from previously existing rocks or fragments of extinct organisms. On the Earth’s surface, deposits build up and cause them to form. The layering or bedding of sedimentary rocks is frequently distinctive.
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 takes place under water or at least in the presence of water.
FORMATION OF Sedimentary Rocks
Sedimentary rock formation is a persistent phenomenon. The so-produced sediments are carried to the settling basins, such as the sea floor, where they are deposited, get compacted and consolidated, and then are finally changed into a cohesive solid mass. That rock is made of sediment.
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.
Sedimentary rocks are broadly grouped into three classes on the basis of their mode of formation: Mechanically formed or Clastic Rocks; Organically formed Rocks and Chemically formed Rocks
The last two groups are considered as a single class and named as Non-Clastic Rocks. Clastic ( Mechanically Formed) Rocks . A series of well-defined steps are involved in the formation of clastic rocks.
Decay and Disintegration
Natural forces like the atmosphere, water, and ice acting on rocks on the surface of the earth expose them to deterioration and disintegration.
The original hard and coherent rock bodies are gradually broken down into smaller and still smaller fragments, grains and particles.
Transport of Sediments
Sedimentary rocks are formed from the detritus left over after the deterioration and disintegration of older rocks, but this material must first be transported to an appropriate location before it can once again be transformed 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.
Transported to settling basins are the sediments created by weathering and erosion. These basins could be found in a variety of settings, including deep sea environments, seashores, and continents.
Sedimentary rocks that were created in various environments will therefore exhibit unique inherent characteristics.
The glacial deposits, fluvial deposits, glacio-fluvial deposits, and eolian deposits, each of which gives rise to a specific type of sediment accumulation, may all be present in continental environments.
Some sediments in marine deposits may be dumped right next to the shore, at a shallow depth in the ocean, or miles away in a deep-sea environment.
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: welding or cementation.
Welding is the process of compaction of the sediments accumulated in lowerlayers 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.
Cementation is the process by which loose grains or sediments in a settlementbasin 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.
Chemically Formed (Non-clastic) Rocks
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 maybe 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 lime stones, rock salt, gypsum, and anhydite.
Organically Formed (Non-clastic} Rocks
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.
The death and decay of these organisms within the water bodies gradually results in huge accumulations of carbonate materials, which get compacted and consolidated in the same manner as the normal sediments.
Lime stones are the best examples of organically formed sedimentary rocks
(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)Shape of Grains
The sediments making the rocks may be of various shapes: rounded, sub rounded, angular and sub angular.
They may show spherecity to various degrees.
Roundness and spherecity 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 elongate 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.
IMPORTANT SEDIMENTARY ROCKS
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 2mm average diameter but some times 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:
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.
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
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 2mm. 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.
On the basis of the dominant grade of the constituent gravels in following three types:
Boulder-ConglomeratesCobble-Conglomerate Pebble-Conglomerate (gravels> 256mm) (gravels: 64-256 mm) (gravels: 2-64 mm)
On the basis of source of the gravels, as
(i) Basal-conglomerates Having gravels derived from advancing sea-waves over subsiding land masses;
(ii) Glacial-conglomerates In which gravel making the conglomerates are distinctly ofglacial origin;
(iii) Volcanic-conglomerates In which gravels are of distinct volcanic origin but havesubsequently been subjected to lot of transport resulting in their smoothening and polishing by river transport before their deposition and compaction or cementation.
On litho logical basis
(a) Oligomictic Simple in composition, these gravels are made up ofquartz, chert and calcite;
(b) Polymictic. In these conglomerates the constituent gravels are derived from rocks ofall 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.
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 2mm and 1/16 mm. Silica in the form of very resistant mineral QUARTZ is the dominant mineral constituent of most sandstones.
Quartz (Si02) 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 sand stones composition.
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:
Type: Coarse-grain Medium-grain Fine -grain
Size-range: 2 mm-l/2 mm 1/2 mm-1/4 mm 1/4 mm-l/16 mm
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.
On the basis of their composition and the nature of the cementing material.
Silica (Si02) 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.
Calcareous Sandstones. are those varieties of sandstones in which carbonates of calcium and magnesium are the. cementing materials.
Argillaceous Sandstones These are among the soft varieties of sandstone because the cementing material is clay that has not have much inherent strength.
Ferruginous Sandstones As the name indicates, the cementing material is an iron oxide compound. On the basis of mineralogical composition
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
respe ctivel y.
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.
It is a variety of sandstone that is exceptionally rich in mica dispersed in parallel or sub parallel 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.
Ganister. 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.
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.
Average Chemical Composition of Shales
S.N Oxide % age S.No Oxide %age
1 – 58% 5 Ca 3%
2 Si02 15% 6 O 3%
3 Al20 6% 7 K2 1%
4 2% 8 5%
Shales have been classified variously. Three Classes On The Basis Of Their Origin:
on the basis of their mineralogical composition:
Quartz shales: rich in free quartz content.
Felspathic shales: in which felspars and clay minerals predominate; silica becomes asecondary constituent. Chloritic shales: in these shales, minerals of chlorite group and clay-group make the bulk of the shales. Micaceous shales: these are rich in muscovite mica and other flaky and play minerals.
These are formed from decay and decomposition of pr-existing rocks followed by compaction and consolidation of the particles in adjoining basins without much mixing;.
These are deposits of clastic materials of finer dimensions transportedover wide distances before final settlement in basins of deposition.
In such shales, materials derived both from clastic sources and non clasticespecially those from organic sources make up the rock.
on the basis of their mineralogical composition:
Quartz shales: rich in free quartz content.
Felspathic shales: in which felspars and clay minerals predominate; silica becomes a secondary constituent. Chloritic shales: in these shales, minerals of chlorite group and clay-group make the bulk of the shales. Micaceous shales: these are rich in muscovite mica and other flaky and play minerals.