In this article we will discuss about the classification of sedimentary rocks on the basis of the nature of sediments:- 1. Mechanically Formed Sedimentary Rocks 2. Chemically Formed Sedimentary Rocks 3. Organically Formed Sedimentary Rocks.
1. Mechanically Formed Sedimentary Rocks:
Previously formed rocks are subjected to mechanical or physical disintegration and thus the rocks are broken into fragments of different sizes. These are called fragmental rock materials or clastic materials which become source materials for the formation of clastic sedimentary rocks. These materials are obtained, transported and deposited at suitable places by different exogenous processes (geological agents) like running water (rivers), wind, glaciers, and sea waves.
These materials are further broken down into Finer particles due to their mutual collision during their transportation. These materials after being deposited and consolidated in different water bodies (sedimentation basins, lakes, seas, rivers etc.) form sedimentary rocks known as clastic sedimentary rocks. Sandstones, conglomerates, silt, shale, clay etc., are important members of this group.
(a) Sandstones are formed mostly due to deposition, cementation and consolidation of sand grains. Sand grains are divided into five categories on the basis of their size. When sand grains are deposited in water bodies and are aggregated and consolidated by cementing elements (e.g., silica, calcium, iron oxide, clay etc.), sandstones are formed.
The colour of sandstones varies according to the nature and amount of cementing elements and minerals. Sandstones become red or gray when cemented by iron oxides but these become white or gray when calcium carbonate dominates. Sandstones become hard and resistant to erosion when cemented by silica. On an average sandstones are porous rocks and water easily percolates through them.
On the basis of textural and mineralogical characteristics sandstones are classified into:
(i) Quartz arenites (arenite from Latin word arena, meaning thereby sand) composed entirely of quartz grains,
(ii) Arkose sandstones (feldspar being the dominant mineral),
(iii) Lithic arenites (composed of fine grained rock fragments, mostly derived from shales, slates, schists and volcanic rocks) and
(iv) Greywacke’s sandstones (composed of quartz, feldspar and rock fragments surrounded by a fine-grained clay matrix).
(b) Conglomerates are formed due to cementation and consolidation of pebbles of various sizes together with sands. The term conglomerate is applied to cemented fragmental rocks containing rounded fragments such as pebbles and boulders; if the fragments are angular or subangular, the rock is called breccia’.
Polished and rounded fragments are called pebbles having a diameter up to 4-64 mm while those fragments which have the diameter up to 256 mm or more are called boulders. The rock fragments after being cemented by clay form gravels. Though gravels are found in layers but there is general absence of uniformity. When the rounded fragmental materials are cemented by quartz, the resultant rocks become conglomerates. If conglomerates are formed due to their cementation by silica, they become very hard rocks and resistant to erosion.
(c) Clay rock and shale:
Clay rocks are formed due to deposition and cementation of fine sediments. The rocks formed of the sediments having the grain size of 0.0312 mm to 0.004 mm are called silts whereas clays are formed when the sediments of the grain size of0.004 mm to 0.00012 mm are cemented and consolidated. Sill and clay are soft and weak rocks but they are definitely impervious.
Clay rocks are formed exclusively of kaolin minerals. Since clay rocks are not soluble and hence these are least affected by chemical weathering but these are easily eroded away. Pure clay rocks are of white colour but they change in colour when they are mixed with the impurities of other materials. Shales are formed due to consolidation of silt and clay. Shales are formed of thin laminae which are easily separated. Shales are impermeable rocks and therefore they hold mineral oil above them.
2. Chemically Formed Sedimentary Rocks:
Running water contains chemical materials in suspension. When such chemically active water comes in contact with the country rocks in its way, soluble materials are removed from the rocks. Such materials are called chemically derived or formed sediments. These chemical materials after being settled down and compacted and cemented form chemical sedimentary rocks such as gypsum and salt rocks.
3. Organically Formed Sedimentary Rocks:
The sediments derived from the disintegration or decomposition of plants and animals are called organic sediments. These sediments after being deposited and consolidated form organic sedimentary rocks.
On the basis of lime and carbon content these rocks are divided into 3 categories e.g.:
(a) Calcareous rocks,
(b) Carbonaceous rocks, and
(c) Siliceous rocks.
(a) Calcareous rocks are formed due to deposition and consolidation of sediments derived from the skeletons and remains of those animals and plants which contain larger portion of lime. Limestone is the most significant characteristic example of calcareous rocks.
Limestones are formed in the following manner:
(i) Calcium oxide (CaO) reacts with water (H2O) to form calcium hydroxide (Ca (OH)2)
CaO + H2O → Ca(OH)2)
(ii) Calcium hydroxide reacts with carbon dioxide (CO2) to form calcium carbonate (CaCO3)
Ca(OH)2 + CO2 →CaCO3 + H2O
The calcareous rocks are collectively called as carbonate rocks or simply carbonates. Lime stones (CaCO3) or calcium carbonate, magnesium carbonate, (MgCO3) and dolomite (CaMg (CO3)2) are important carbonate rocks. Limestones are found in both the forms-thinly bedded and thickly bedded.
They are formed of both fine sediments as well as coarse sediments. The most dominant minerals are calcite (of hexagonal shape) and argonite (of orthorhombic shape).
Since limestones are formed of chemically soluble materials and hence these are most susceptible to chemical weathering as follows:
(i) Carbon dioxide (CO2) after being dissolved in water form carboric acid (H2 CO3).
CO2 + H2O ↔ H2 CO3
(ii) Carbonic acid reacts with limestone (CaCO3) to form calcium bi-carbonate (Ca (HCO3)2)
H2CO3 + CaCO3 → Ca (HCO3)2
Though limestones are very weak rocks in humid regions because these easily dissociate when they come in contact with water but these become resistant rocks in hot and dry climate because of the fact that limestones have uniform and homogeneous structure and hence these are not affected by differential expansion and contraction due to temperature changes.
The rocks having the carbonates of both calcium and magnesium are known as dolomites which are less soluble than limestones. These carbonate rocks, after weathering and chemical erosion, give birth to karst topography. Chalk is another form of carbonate rocks but it is softer and more porous than limestone. Chalks are formed due to precipitation of carbonate materials which are derived from micro-organisms like foraminifera.
(b) Carbonaceous rocks are dominated by carbonic materials which represent vegetation remains. These rocks are formed due to transformation of vegetations because of their burial during earth movements and consequent weight and pressure of overlying deposits. The initial form of carbonaceous rocks is peat which is of a dark gray colour.
Vegetation remains can be seen with the help of microscope. The other subsequent forms of carbonaceous sedimentary rocks are lignite, bituminous and anthracite coals with greater proportion of carbon and darker colour. Coals are also found in stratified form wherein coal layers are known as coal seams. Carbonaceous rocks are more important economically than geomorphologically.
(c) Siliceous rocks are formed due to dominance of silica content. Siliceous rocks are formed due to aggregation and compaction of wastes derived from sponge and radiolarian organisms and diatom plants. Geysreites are also deposits of silica around geysers. Geyserites have different colours e.g., white, gray or pink due to impurities of deposition of various types of sediments.
Classification on the Basis of Transporting Agents:
Sedimentary rocks are also classified on the basis of transporting agents or geological agents (e.g. running water or rivers, wind, glaciers, oceanic currents and sea waves). These agents of transportation obtain different types of sediments and deposit them in suitable places where sediments are consolidated and cemented to form sedimentary rocks of various sorts.
Based on major transporting agents, sedimentary rocks are divided into:
2. Aeolian and
3. Glacial rocks.
(1) Argillaceous rocks are also called as aqueous rocks because these are formed in water areas. Aqueous word has been derived from Latin world ‘aqua’ which means ‘water’. Aqueous rocks are called argillaceous rocks because of the dominance of clay in the rocks. In fact, the word argillaceous has been derived from Latin word ‘argyll’ or ‘argill’ meaning thereby clay.
Argillacous rocks are characterized by their general softness. These are essentially impervious rocks.
Argillaceous rocks are further divided into 3 sub-types on the basis of the places of their formation:
(i) Marine argillaceous sedimentary rocks are formed due to deposition and consolidation of sediments in the oceans and seas mainly in their littoral zones. The process of sedimentation in marine environment is well ordered and sequential in character. In other words, the size of particles deceases progressively from the coastal lands towards the seas or the oceans e.g., the order of the particles from the coast lands towards the sea is of boulders, cobbles, pebbles, granules, sands, silts, clay and lime. It is evident that as we go away from the coast lands towards the sea, the size of sediments becomes so fine that they are kept in suspension with oceanic water. Sandstones, limestones, dolomites and chalk are the most important examples of marine argillaceous sedimentary rocks.
(ii) Lacustrine argillaceous sedimentary rocks are formed due to deposition and consolidation of sediments in Lake Environment. Generally, the sediments are deposited at the floor of the lakes.
The lacustrine rocks may be seen in 3 conditions viz.:
(a) If the lake becomes dry,
(b) If the floor of the lakes is raised due to earth movements, and
(c) If the whole lake is filled up with sediments.
It may be pointed out that there is no ordering in the size of sediments as is the case with the seas and the oceans.
(iii) Riverine argillaceous sedimentary rocks are those which are formed due to deposition of sediments in the riverine environment. The sediments may be deposited in the beds of the rivers and in the flood plains. Such deposition includes alluvia which are dominated by clay. Alluvia are deposited on either side of the alluvial rivers during floods. It may be pointed out that alluvial deposits are renewed almost every year. Alluvial deposits develop polygonal cracks due to their exposure to insolation.
(2) Aeolian sedimentary rocks are formed due to deposition of sands brought down by the wind. Preexisting rocks are greatly disintegrated due to mechanical weathering in the hot and dry regions. This process results in the formation of immense quantity of sands of different sizes. Winds pick up these sands and deposit them at various places. The particles are further comminuted into finer particles due to attrition while they are being transported from one place to another.
Continuous deposition of sands results in the formation of different layers but these layers are not well consolidated as is the case with the argillaceous rocks. Sometimes, there is complete absence of layers in the airborn or aeolian sedimentary rocks. Loess is the most important member of this group. Loess is, in fact, the heaps of unconsolidated fine materials.
There is general absence of laminae and layers in the loessic formation. These are soft and porous rocks. Water can easily infiltrate in the loessic deposits. Thus, loess is easily eroded away. Thus, most outstanding characteristic feature of loess is that the entire loessic mass may stand like a vertical cliff or wall. The best example is observable on the left and right banks of the palaeochannel and valley of the Narmada River at Dhunwadhar falls (Bheraghat) near Jabalpur (M.P.) where the loessic banks rise 20 to 25 m from the valley floor and form complete vertical free- face cliff section.
The sediments are so loosely arranged that they can be removed even by using fingers. The most extensive loessic deposits are found in north China where the thickness of sediments is of several hundred metres. The deposits are of yellow colour and are rich in lime and hence these look like fine loam soils.
The Yellow river (formerly Hwang Ho) and its tributaries easily erode the loessic deposits and hence the river becomes overloaded and causes frequent severe floods. It may be pointed out that the Yellow river of China carries the largest amount of sediments (1640 million tonnes per year) in the world. The river is called “Yellow” because of the yellow colour of the sediments which are derived through the erosion of yellow coloured Chinese loess.
(3) Glacial rocks:
The materials deposited by glaciers are called glacial drifts which are deposited in four conditions and therefore there are four types of morainic deposits viz.:
(i) Lateral moraines, when glacial materials are deposited on either side of a glacier,
(ii) Medial moraines, when glacial sedimentary materials are deposited along the joining glaciers (the lateral moraines of joining ice streams merge and form a single medial moraine in the middle of larger flows),
(iii) Ground moraines, when the glacial materials are deposited in the bed of the glacier, and
(iv) Terminal moraines (these are formed when the glacier is ablated and materials are deposited therein (fig. 8.15).